scripts/get_maintainer.pl: add patch/file search for keywords

Based on an idea from Wolfram Sang.

Add search for MAINTAINERS line "K:" regex pattern match in a patch or file
Matches are added after file pattern matches
Add --keywords command line switch (default 1, on)
Change version to 0.21

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>