Before this commit, we were not supporting t:Path.t/0 paths but rather
t:String.t/0 paths in EEx file-related functions.
With this commit, we're not supporting t:Path.t/0. It's mostly calling
IO.chardata_to_string/0 on the given filename.
Another thing that was fixed is that now
:file and :line options in file-related EEx functions default to the
given file and line, but can be overridden. Before, they were forced.
I also cleaned up the docs here and there and added more examples.