Allow management of PO files

[bgaillard]

Their is an issue to manage PO files here https://github.com/airbnb/polyglot.js/issues/30.

With the implementation of #4 we could be able to add a custom PO parser, the following implementations could be useful :

• https://github.com/andris9/gettext-parser for example, this one will only work with NodeJS and not in the browser ;
• https://github.com/rubenv/pofile, this one works in the browser, but perhaps it could be useful to parse MO files on browser (don't knwo how to do this for now).

[Phyks]

FWIW, I know about https://github.com/fabi1cazenave/webL10n which is using *.properties files (with an already implemented parser and so on).

And the format used in this lib is really close to what Airbnb is doing in their lib. So it may be easier to implement :)