Choose syntax from file extension, not content.

Right now the Sass Converter decides syntax [based on content](https://github.com/jekyll/jekyll-sass-converter/blob/master/lib/jekyll/converters/sass.rb#L33) This is unreliable since comments can contain any characters, for example:

```sass
// Some grid framework
// TL;DR it's a grid framework
// Ok, so here's the deal about this grid, blah blah...

*
  box-sizing: border-box;
```

This is valid Sass but will be parsed as Scss by Jekyll's sass-converter since it contains a `;` character.

### My Solution

I wish Converters gave you access to the path of the file being converted. If they did, we could simply match on the path and choose the right extension. Since it doesn't I have split the Converter into two; one which matches `scss` files and contains all the converter code, and one which extends the `scss` converter but matches on `sass` files.

**Unfortunately** I think the tests must be rewritten, one for each converter. This is something I don't know how to do, but I've introduced a failing test to get that started.