Jekyll source validation

[html-proofer](https://github.com/gjtorikian/html-proofer) is the most commonly used validator of jekyll sites, and it operates on the rendered version of the site.

By combining various existing coala plugins, and improving the YAML bears, coala could do equivalent validation on the source version of the site.

An important aspect of the YAML validation is ensuring that it can be round-tripped. i.e. read into a library, modified, and emitted again with only the modifications being in the diff.

https://pypi.python.org/pypi/ruamel.yaml is one attempt at doing this, but it often requires overwriting the reader and writer to ensure round-tripping.

https://github.com/nexB/saneyaml is a new attempt at doing this.

A possible mentor of this project is @pombredanne

Start by comparing html-proofer's rendered-site validation with the existing coala plugins and YAML bears mentioned in the issue. Review ruamel.yaml and saneyaml for round-tripping behavior; done means source validation is covered and YAML changes can be read, modified, and emitted with only intended differences.

Written by the indexing model from the issue text.