Add more flexibility to abi checking.

1) Marking a field "required" robs us of flexibility. Make fields optional
allows us to update the message format without needing to necessarily
update the reference abi-dumps.

2) Allow local warnings with the invocation of header-abi-diff.

Test: Added a function to libjpeg, got a warning claiming abi extended.

3) Introduce a flag which allows us to not filter any abi. This is
useful for cases when libraries do not include what they export. eg :
libsqlite.

Test: abi-dump size of libsqlite:
	without no-filter : 0
	with no-filter : ~9M

Change-Id: I6cfeacc8711f6df7a4136c2a27b5638988a2c54b