Handle unsupported extension field types more gracefully.

Handle unsupported extension field types more gracefully.

Previously the generator would stop with NotImplementedException as
soon as a required or repeated extension field is found. New behaviour
is to just ignore the unsupported field and note that in a comment
in the generated file.

Furthermore, allow skipping of extension fields using the generator
option (nanopb).type = FT_IGNORE.

Update issue 83
Status: FixedInGit