Make it possible to run poabc and gtgrep on podiffs

Moved to: https://github.com/pyg3t/pyg3t/issues/25

It would be nice to be able to run poabc and gtgrep directly on the podiffs because that would make it possible to only look at the part of the files that has actually been altered.

One possible, and relatively simple implementation could involve detecting that the file is a podiff in gtparse, then create a dummy po file by only including the unaltered lines and the new lines from the diff, and then parse that in the usual way. The downside of this idea is that we will probably loose the linenumber information.