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.
Update WikiCloth to 0.8.1 and support footnotes
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.
Merge pull request #356 from repotag/escape_rest2html_filepath

Properly escape the filepath to rest2html
Merge pull request #338 from Razer6/preserve_encoding

Force encoding of posix_spawn output
Merge pull request #306 from leandro-lucarella-sociomantic/field-name-limit

rst2html: Extend the field_name_limit to 50
Merge pull request #317 from whym/wikicloth-footnotes

Update WikiCloth to 0.8.1 and support footnotes
Perform case-insensitive test for file extensions
Merge remote-tracking branch 'origin/master' into pchaigno-rmarkdown

* origin/master:
  Escape the filepath to rest2html so paths containing spaces will be passed to CommandImplementation#execute correctly.
  Update README.md
  Tables are cooler
  Organize the README's whitelisted elements a bit better
  Force encoding of posix_spawn output
  Force encoding of posix_spawn output
  Update WikiCloth to 0.8.1 and support footnotes
  rst2html: Extend the field_name_limit to 50
Case-insensitive on just markdown
Merge pull request #358 from github/pchaigno-rmarkdown

Use case-insensitive regex for file extension matching.