Refactor the `chooseModuleName` function.

This changes it so it uses a single Regexp, instead of its previous
slightly more imperative logic. Nothing should change, overall, though
this could be a little faster.

The regexp `\/-([^\/]*)$` may be de constructed as follows:
- `\/-` **matches a slash followed by a dash**
- `([^\/]*)` **matches a group of any number of any character persides a
  slash
- `$` **matches the end of the string, so only the last 'part'
  (supposing parts are separated by slashes) will be matched

The `replace` call then, _matches the last 'part' beginning with a dash_
and then replaces it with the expression '/_$1'. In other words, the
supposedly matched slash, an underscore (which takes the place of the
dash) and the group matched between the dash and the end of the string
'$1'.

So:
```javascript
'something/else/-wat'.replace(/\/-([^\/]*)$/, '/_$1');
// 'something/else/_wat'
```
Merge pull request #47 from yamadapc/master

Very light refactor to the `chooseModuleName` function.