* Fix ansible-doc traceback when a plugin doesn't parse correctly
* Change extract_metadata ivocation to take either an ast or source
code. When given source code, it can find file offsets for the start
and end of dict. When given the ast, it is quicker as it doesn't have
to reparse the source. Requires changing the call to the function to
use a keyword arg.
* Fix reading of metadata to find the last occurrence of
ANSIBLE_METADATA instead of the first.
* Add some more unittests to get closer to complete coverage
* Unittests for extracting metadata from plugins
* Port plugin_docs to use the generic extract_metadata function
* Make the helper functions seek_end_of{string,dict} private
