Implement `--ascii` (`writerPreferAscii`) in writers, not App.

Now the `write*` functions for Docbook, HTML, ICML, JATS,
Man, Ms, OPML are sensitive to `writerPreferAscii`.  Previously
the to-ascii translation was done in Text.Pandoc.App, and
thus not available to those using the writer functions
directly.

In addition, the LaTeX writer is now sensitive to
`writerPreferAscii` and to `--ascii`.  100% ASCII
output can't be guaranteed, but the writer will use
commands like `\"{a}` and `\l` whenever possible,
to avoid emiting a non-ASCII character.

A new unexported module, Text.Pandoc.Groff, has been
added to store functions used in the different groff-based
writers.

1 files changed, 45 insertions, 0 deletions

diff --git a/test/command/ascii.md b/test/command/ascii.md
new file mode 100644
index 000000000..523baa46c
--- /dev/null
+++ b/test/command/ascii.md
@@ -0,0 +1,45 @@
+```
+pandoc -t html --ascii
+äéıå
+^D
+<p>&#228;&#233;&#305;&#229;</p>
+```
+
+```
+pandoc -t latex --ascii
+äéıå
+^D
+\"{a}\'{e}\i \r{a}
+```
+
+```
+pandoc -t man --ascii
+äéıå
+^D
+.PP
+\[u00E4]\[u00E9]\[u0131]\[u00E5]
+```
+
+```
+pandoc -t ms --ascii
+äéıå
+^D
+.LP
+\[u00E4]\[u00E9]\[u0131]\[u00E5]
+```
+
+```
+pandoc -t docbook --ascii
+äéıå
+^D
+<para>
+  &#228;&#233;&#305;&#229;
+</para>
+```
+
+```
+pandoc -t jats --ascii
+äéıå
+^D
+<p>&#228;&#233;&#305;&#229;</p>
+```