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.

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