Add option for returning result as PO-formatted string

Whereas the [GNU xgettext program](http://www.gnu.org/savannah-checkouts/gnu/gettext/manual/html_node/xgettext-Invocation.html) generates a PO-formatted file, xgettext-js currently only returns a JavaScript object. This allows for more flexibility in consuming matched strings, but also places the burden on the end-user to convert said data to the PO format if that is the desired result. Adding an optional configuration property for specifying output format will provide a solution to this need.

The issue names no implementation files or tests; start by locating the configuration handling and the code that currently returns the JavaScript object. Compare the expected output with the linked GNU xgettext PO format, then add coverage showing that the optional format setting returns valid PO output without changing the default.

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