Add `empty_paragraphs` extension.

* Deprecate `--strip-empty-paragraphs` option.  Instead we now
  use an `empty_paragraphs` extension that can be enabled on
  the reader or writer.  By default, disabled.

* Add `Ext_empty_paragraphs` constructor to `Extension`.

* Revert "Docx reader: don't strip out empty paragraphs."
  This reverts commit d6c58eb836f033a48955796de4d9ffb3b30e297b.

* Implement `empty_paragraphs` extension in docx reader and writer,
  opendocument writer, html reader and writer.

* Add tests for `empty_paragraphs` extension.

3 files changed, 99 insertions, 8 deletions

diff --git a/test/command/2649.md b/test/command/2649.md
index af84693c4..8f594cfe1 100644
--- a/test/command/2649.md
+++ b/test/command/2649.md
@@ -88,20 +88,17 @@
 </tr>
 <tr class="even">
 <td><p>1</p></td>
-<td>
-<p><a href="Sébastien_Loeb" title="wikilink">Sébastien Loeb</a></p></td>
+<td><p><a href="Sébastien_Loeb" title="wikilink">Sébastien Loeb</a></p></td>
 <td><p>78</p></td>
 </tr>
 <tr class="odd">
 <td><p>2</p></td>
-<td>
-<p><strong><a href="Sébastien_Ogier" title="wikilink">Sébastien Ogier</a></strong></p></td>
+<td><p><strong><a href="Sébastien_Ogier" title="wikilink">Sébastien Ogier</a></strong></p></td>
 <td><p>38</p></td>
 </tr>
 <tr class="even">
 <td><p>10</p></td>
-<td>
-<p><a href="Hannu_Mikkola" title="wikilink">Hannu Mikkola</a></p></td>
+<td><p><a href="Hannu_Mikkola" title="wikilink">Hannu Mikkola</a></p></td>
 <td><p>18</p></td>
 </tr>
 </tbody>
diff --git a/test/command/3494.md b/test/command/3494.md
index 534041246..249973fb3 100644
--- a/test/command/3494.md
+++ b/test/command/3494.md
@@ -25,8 +25,7 @@
 <td style="text-align: left;">thank you</td>
 </tr>
 <tr class="odd">
-<td style="text-align: right;">
-<p><em>blah</em></p></td>
+<td style="text-align: right;"><p><em>blah</em></p></td>
 <td style="text-align: left;"><em>blah</em></td>
 <td style="text-align: left;"><em>blah</em></td>
 </tr>
diff --git a/test/command/empty_paragraphs.md b/test/command/empty_paragraphs.md
new file mode 100644
index 000000000..3064d3f7d
--- /dev/null
+++ b/test/command/empty_paragraphs.md
@@ -0,0 +1,95 @@
+```
+% pandoc -f native -t docx | pandoc -f docx -t native
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+[Para [Str "hi"]
+,Para [Str "lo"]]
+```
+
+```
+% pandoc -f native -t docx+empty_paragraphs | pandoc -f docx -t native
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+[Para [Str "hi"]
+,Para [Str "lo"]]
+```
+
+```
+% pandoc -f native -t docx | pandoc -f docx+empty_paragraphs -t native
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+[Para [Str "hi"]
+,Para [Str "lo"]]
+```
+
+```
+% pandoc -f native -t docx+empty_paragraphs | pandoc -f docx+empty_paragraphs -t native
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+[Para [Str "hi"]
+,Para []
+,Para []
+,Para [Str "lo"]]
+```
+
+```
+% pandoc -f native -t html5
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+<p>hi</p>
+
+
+<p>lo</p>
+```
+
+```
+% pandoc -f native -t html5+empty_paragraphs
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+<p>hi</p>
+<p></p>
+<p></p>
+<p>lo</p>
+```
+
+```
+% pandoc -f html+empty_paragraphs -t native
+<p>hi</p>
+<p></p>
+<p></p>
+<p>lo</p>
+^D
+[Para [Str "hi"]
+,Para []
+,Para []
+,Para [Str "lo"]]
+```
+
+```
+% pandoc -f html -t native
+<p>hi</p>
+<p></p>
+<p></p>
+<p>lo</p>
+^D
+[Para [Str "hi"]
+,Para [Str "lo"]]
+```
+
+```
+% pandoc -f native -t opendocument+empty_paragraphs
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+<text:p text:style-name="Text_20_body">hi</text:p>
+<text:p text:style-name="Text_20_body"></text:p>
+<text:p text:style-name="Text_20_body"></text:p>
+<text:p text:style-name="Text_20_body">lo</text:p>
+```
+
+```
+% pandoc -f native -t opendocument
+[Para [Str "hi"], Para [], Para [], Para [Str "lo"]]
+^D
+<text:p text:style-name="Text_20_body">hi</text:p>
+<text:p text:style-name="Text_20_body">lo</text:p>
+```