Fix powerline-lint tests

Some notes on the commit:

1. As external_ip and email_imap_alert segments seem to be removed corresponding
   segment_data keys also were removed.
2. Various files that import vim module now have the usual workaround that sets
   vim local to dummy object on error.
3. Syntastic check was failing because it did not specify which highlighting
   groups it uses in documentation. I made it specify them and also moved format
   strings to keywords. Same for some other plugin-local themes.
4. powerline-lint script got --debug argument. Pretty useless currently though:
   it only makes it output traceback for ImportError when failing to import
   module to stderr.
5. Moved themes/vim/plugin/\*.json to themes/vim/plugin_\*.json.
6. Fixed powerline-lint that ignored problems from values.
7. Also removed "default_module" key that pointed to inexistent `powerline.segments.plugin.gundo`.