rst2html: Extend the field_name_limit to 50

Field lists are rendered weirdly in reST if the field name is longer
that 14 characters. In this case the table cell where the field name is
gets a colspan=2.

For example this:

```rest
:123456789012345: too long, colspan=2
:12345678901234: OK
```

Gets translated to:

```html
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr class="field"><th class="docinfo-name"
colspan="2">123456789012345:</th></tr>
<tr class="field"><td>&nbsp;</td><td class="field-body">too long,
colspan=2</td>
</tr>
<tr class="field"><th class="docinfo-name">12345678901234:</th><td
class="field-body">OK</td>
</tr>
</tbody>
</table>
```

Increasing this default value of 14 to something more reasonable, like
50, seems sensible. Even when this is just another arbitrary value,
since GitHub have a fixed total width, it seems sensible to not make it
much more longer, since if the field name is too long it will look weird
anyway.
Fix a spelling mistake

Add an 'i' to "santization".
Merge pull request #319 from neven-sajko/spellcheck

Fix a spelling mistake
Force encoding of posix_spawn output
Force encoding of posix_spawn output
Render RMarkdown (.rmd) as Markdown
Organize the README's whitelisted elements a bit better
Tables are cooler
Merge pull request #350 from github/tidy-up-the-readme

Tidy up the readme
Update README.md
Escape the filepath to rest2html so paths containing spaces will be passed to CommandImplementation#execute correctly.
Makes github/markup compatible with JRuby.
Implement assert_html_equal.
Add nokogiri and nokogiri-diff to gemspec
Trying to test jruby on travis.
Restore descriptive error message for test failures
Add jruby to travis.
Skip test_raises_error_if_command_exits_non_zero on JRuby until https://github.com/jruby/jruby/issues/1947 is resolved.
Refactor changes to be ignored in diff. Fix bug that incorrectly ignored '-' changes.
Wrap shell command in a script as a workaround for https://github.com/jruby/jruby/issues/1947.