Add binary executable for extracting strings from keyword functions

This should closely mimic the xgettext [GNU program parameters](http://www.gnu.org/savannah-checkouts/gnu/gettext/manual/html_node/xgettext-Invocation.html), and perhaps include the option for specifying output format (i.e. PO, JSON).

The issue does not name a file, test, or entry point to start from. Read the linked GNU xgettext invocation parameters and inspect the repository to determine the executable's scope; the work is done when the agreed parameters and output formats are implemented and verified.

Written by the indexing model from the issue text.