Create functions in inline to open git blobs

Adds support for reading and inlining inputs of latex documents in the
git tree. GitPython (0.3) is used to access git blobs. Blobs are encoded
as utf-8 unicode text objects.
Create latexdiff command

This command is a pipeline for running latexdiff between the current
manuscript, and a revision of the manuscript in git. Any SHA fragment
can be passed. GitPython is used to read git blobs.

Documents are are included with \input{} commands are recursively
inlined to support latexdiff.

The command then runs latexdiff and latexmk to compile the difference
document.

Updated README for preprint diff