Benchmark improvements

Hello,

This is just a tiny pull request that enhance a bit our benchmarking. The benchmark-ips gem seems to be more adequate as we don't have to wait too long to get feedback from other parsers (e.g. Kramdown which takes around 6 minutes to parse 10 thousands times the same document).

I also think that we should move the `fixture.text` file to a `fixtures` folder as we may have other fixtures in the future plus the name was not so descriptive.

Have a nice day.