Rationalized behavior of --no-tex-ligatures and --smart.

This change makes `--no-tex-ligatures` affect the LaTeX reader
as well as the LaTeX and ConTeXt writers.  If it is used,
the LaTeX reader will parse characters `` ` ``, `'`, and `-`
literally, rather than parsing ligatures for quotation marks
and dashes.  And the LaTeX writer will print unicode quotation
mark and dash characters literally, rather than converting
them to the standard ASCII ligatures.

Note that `--smart` has no affect on the LaTeX reader.

`--smart` is still the default for all input formats when
LaTeX or ConTeXt is the output format, *unless* `--no-tex-ligatures`
is used.

Some examples to illustrate the logic:

```
% echo "'hi'" | pandoc -t latex
`hi'
% echo "'hi'" | pandoc -t latex --no-tex-ligatures
'hi'
% echo "'hi'" | pandoc -t latex --no-tex-ligatures --smart
‘hi’
% echo "'hi'" | pandoc -f latex --no-tex-ligatures
<p>'hi'</p>
% echo "'hi'" | pandoc -f latex
<p>’hi’</p>
```

Closes #2541.

1 files changed, 14 insertions, 10 deletions

diff --git a/README b/README
index 192415199..50ec12596 100644
--- a/README
+++ b/README
@@ -374,7 +374,7 @@ Reader options
     `...` to ellipses. Nonbreaking spaces are inserted after certain
     abbreviations, such as "Mr." (Note: This option is selected automatically
     when the output format is `latex` or `context`, unless `--no-tex-ligatures`
-    is used.)
+    is used.  It has no effect for `latex` input.)
 
 `--old-dashes`
 
@@ -651,15 +651,19 @@ Options affecting specific writers
 
 `--no-tex-ligatures`
 
-:   Do not convert quotation marks, apostrophes, and dashes to
-    the TeX ligatures when writing LaTeX or ConTeXt. Instead, just
-    use literal unicode characters. This is needed for using advanced
-    OpenType features with `xelatex` and `lualatex`. Note: normally
-    `--smart` is selected automatically for LaTeX and ConTeXt
-    output, but it must be specified explicitly if `--no-tex-ligatures`
-    is selected. If you use literal curly quotes, dashes, and ellipses
-    in your source, then you may want to use `--no-tex-ligatures`
-    without `--smart`.
+:   Do not use the TeX ligatures for quotation marks, apostrophes,
+    and dashes (`` `...' ``, ` ``..'' `, `--`, `---`) when
+    writing or reading LaTeX or ConTeXt.  In reading LaTeX,
+    parse the characters `` ` ``, `'`, and `-` literally, rather
+    than parsing ligatures for quotation marks and dashes.  In
+    writing LaTeX or ConTeXt, print unicode quotation mark and
+    dash characters literally, rather than converting them to
+    the standard ASCII TeX ligatures.  Note: normally `--smart`
+    is selected automatically for LaTeX and ConTeXt output, but
+    it must be specified explicitly if `--no-tex-ligatures` is
+    selected. If you use literal curly quotes, dashes, and
+    ellipses in your source, then you may want to use
+    `--no-tex-ligatures` without `--smart`.
 
 `--listings`