add support for parsing filter-type and test-type plugins via yaml files (reopened)

[philfry]

Hi,

reopened #201 because I found that the filter and test documentation provided as yaml are are not parsed yet.

Example:

• plugins/filter/dict_filter.py defines the filters "my_dict_filter_foo", "my_dict_filter_bar" etc.
• plugins/filter/my_dict_filter_foo.yml, plugins/filter/my_dict_filter_bar.yml providing the corresponding documentation

Ansible-doc is able to display the filter and test documentation provided as yaml files, while galaxy-importer ignores them.
This PR adds "yaml" and "yml" extensions to the parseable files.