Added separator option

I had one really annoying problem when using ``grunt-rev`` together with ``grunt-contrib-compress`` and gzip compression.

I configured my compress task to change file extensions of gzip compressed files to ``.css.gz`` using the ``ext`` option:
```
{
  expand: true,
  cwd: 'dist/htdocs/',
  dest: 'dist/htdocs/',
  src: ['**/*.css'],
  ext: '.css.gz' 
}
```

Everything fine - almost. Except that 
my ``59c28e95.test.css`` was renamed to ``59c28e95.css.gz``
my ``17575a8d.vendor.css`` was renamed to ``17575a8d.css.gz``
etc.

I know it's probably a "bug" in compress or in the grunt (or even node) file reader/writer/glob but nevertheless I added an optional ``separator`` option so you can at least workaround this problem by having your files renamed to ``59c28e95-test.css`` instead of ``59c28e95.test.css`` for example (dash instead of dot).

Merge it if you like :)

PS: I know I could have just flipped the execution order of the compress and the rev task but it seemed like a more "reliable" way doing it that way ;)