Alter plugin layout: move *.plugin to *.{ext}.plugin

I do not see why vim plugins should receive such a generic name as
`powerline.segments.plugin.gundo`: taking a name that clearly indicates that
`gundo` is a *vim* plugin is better.
Mention `showtabline` option in Vim tips and tricks
Add information about writing segments

Fixes #629
Ref #287
Add information about shell and ipython local themes
Fix path to __main__ theme
Document how to create Vim local themes
Rearrange renderers: move *_prompt modules one level up

Renames powerline.renderers.zsh_prompt to powerline.renderers.shell.zsh, same
for other *sh_prompt modules.
Improve documentation
Document how to create extensions

Fixes #287
Drop unneeded ignores
Fix some style problems
Ignore E101 (“indentation contains mixed spaces and tabs”)
Mention long lines in CONTRIBUTING.rst
Merge pull request #986 from ZyX-I/developer-documentation

Developer documentation and module layout refactoring