Heuristic rule to detect AsciiDoc files

As reported in #1638, there are conflicts between AsciiDoc and AGS Script files on the `.asc` extension.
This pull request adds an heuristic rule to detect AsciiDoc files.
It tries to find title's syntax in AsciiDoc (`===`).

I tested it against two repositories:
- [vasofilipov/wisp-game](https://github.com/vasofilipov/wisp-game) - AGS Script
```
100.00% AGS Script
```

- [progit/progit2](https://github.com/progit/progit2) - AsciiDoc
```
66.73%  CSS
25.16%  XSLT
7.01%   Ruby
1.10%   AGS Script

Ruby:
Gemfile
Rakefile
book/07-git-tools/git-credential-read-only

AGS Script:
LICENSE.asc
book/toc.asc

CSS:
theme/epub/epub.css
theme/html/html.css
theme/mobi/mobi.css
theme/pdf/pdf.css

XSLT:
theme/epub/epub.xsl
theme/html/html.xsl
theme/mobi/mobi.xsl
theme/pdf/pdf.xsl
```

EDIT: The [Travis build](https://travis-ci.org/github/linguist/builds/39160064) is passing.