Add BaseFs to RenderingContext

Add BaseFs to RenderingContext

The org mode renderer supports including other files [1]. We don't want to
allow reading of arbitrary files (go-org defaults to ioutil.ReadFile [2]) but want
to make use of the FileSystem abstractions hugo provides. For starters we will
allow reading from the content directory only

[1]: e.g. `#+INCLUDE: ./foo.py src python` includes `foo.py` as a python source
block.