sentinel: add optional (pdf|lua|xe) to 'latex' regex

This fixes the sentinel matching latexmk output if using pdflatex.
Currently, latexmk doesn't put 'lua' nor 'xe' in the output but this
helps future-proof it a little.
sentinel: fix incorrect error message if there is nothing to compile

Previously, if a user ran latexmk and there was nothing to compile,
then the error "LatexMk: problems after [0] pages" would appear.
Instead, we now display "LatexMk: nothing to do".
Merge pull request #5 from seanfarley/master

Add {pdf,lua,xe}latex