Add cache to speedup searching process

[Seluj78]

No description provided.

[JulienPalard]

I don't think caching is the clean way to speed up pogrep. I'd better consider enhancing polib performance: On a test run pogrep -r string it took 22s to compute, 19s in polib.py.

18s are spent in polib.py:parse, parse itself calls process (spends 8s in it) and also uses re.search (spend 3s in it).