Adding compression

It might be horribly non-standard, but I think you could make a data-driven test with something like:

```javascript
describe( 'isCompressibleFile', function() {
    [ 'file.js', 'file.css', ... ].forEach(function(filename) {
        it( 'should compress ' + filesname, function() {
            var filePath = path.join( './test/support', filename );
            var stream = fs.createReadStream( filePath );
            var isCompressible = isCompressibleFile( stream );
            expect( isCompressible ).to.equal( true );
        }
    }
} );