mime.extension() fails if the content-type is parameterized #74

Hello,

This used to work in mime:

mime.define({
    'application/x-foo; type=bar': ['baz']
});

assert.equal('baz', mime.extension('application/x-foo; type=bar'));

It no longer does, given the extension logic currently splits off parameters after the ; (it looks like there was a previous bug fix for this given charsets, etc.).

I would have sent a PR, but given the history there I'm not sure what you want to do with it; it sort of feels like you need an API flag that indicates whether you want "exact match" (modulo whitespace) or "parameterized match" or something that does what it does now.