Remove partials_dir functionality

I don't think the `partials_dir` functionality is very useful, and it's set to `''` by default so it isn't even used for anything (though as an accidental side effect it made partials resolve relative to the `source_dir`, which I have preserved here). Better to just avoid the extra complexity and testing and let people who want to create convenience helpers (that's what helpers are for!). 

Note that the original code did not ensure that `partials_dir` was within the `source_dir`, which meant people could set their `partials_dir` any old crazy place, which would likely cause all kinds of problems.

/cc @lolmaus for the defense