Refactor/use jinja package

[clrcrl]

Previously, this package overrode Atom's settings so that .sql, .yml and .md files were automatically rendered as using the Jinja-flavored version of their grammars.

This PR means that users of this package will have to instead change their Atom Config to use the Jinja-flavored grammars for these files.

Making the (formerly) default behavour user-defined makes the behavour of the package more explicit.

I can also then leverage the atom-jinja2 package as a dependency, significantly reducing the amount of code in this repo. Note that I still need to include the .source.sql.jinja grammar as the atom-jinja2 package doesn't support SQL.