New assertion. It matches file content against a regular expression.

Hi. While I was playing with chai-fs I noticed that matching file content against a RegExp would be useful. So here it is.
Two main files 'lib/assertions/content_match.js' and 'test/specs/file_content_match.js' are based on their counterparts 'lib/assertions/content.js' and 'test/specs/file_content.js'. Authoring this files I was complying to TDD, so they just work. Potential flaw is that they have been tested on Windows only. I have read your guide for contributors, but I've got only one machine (with Windows), so please don't kill me. I have also left a trace in README.md. Thanks.