Improve removal of latex comments

Switch to the regex in http://stackoverflow.com/a/13365453

The previous one would delete `}%` rather than just the trailing
comment. This is a problem for the tex produced by the vc package.
Fix argument in diff command
Deal with \InputFileIfExists for inline()

The inline() function will now properly replicate tex-behaviour for
inlining content from \InputIfFileExists, and inputting fall-back
content if it does not.

The inline_blob() function will also deal with \InputIfFileExists,
falling back to reading the file from the file system if it is not in
VC.

closes #7