Added support for options, set slim context to jekyll config, updated readme

I added support to slim options by setting the `slim` key in `_config.yml`.
Added also a context to slim render calls: the context is now the jekyll config hash so it's possible to access configuration data in slim.

**Important note**: I added a dependency on `activesupport` (4.0.0) with the helper `deep_stringify_keys` because it allows to convert all keys of a hash in symbols. Slim only accepts symbol keys as options so I think it was a good idea because anyway activesupport is widely used thanks to rails.