Automatically get version, date for manual

[rchurchley]

As discussed in #81 and #87, we would like to have the manual automatically update the version number and date in \maketitle according to the \ProvidesPackage declaration of beamerthememetropolis.sty. Normally, we'd do this with \fileversion and \filedate after calling \GetFileInfo{beamerthememetropolis}, but unfortunately \GetFileInfo{file} does not work unless file is loaded during the LaTeX run. This is not an option for us since beamerthememetropolis depends on the beamer class.

As it turns out, there is a package to solve this exact problem: readprov provides a command to read a file's metadata without actually loading it. This pull request adopts this solution.