Cache statusline contents in Python

Each window is now tagged with an UUID and this UUID is used to cache
the window's statusline contents in the Python VimRenderer in order to
avoid translating the statusline contents to and from a vimdict
unnecessarily.

Refs #11.
Cleanup code and comments
Minor code cleanup
Cache segment contents after rendering the statusline

Non-current segments used the wrong contents because outdated
information was cached (self.segments was repopulated in the render()
method which was called *after* the contents was cached). Now the
contents are cached after the parent class' render() method is called so
the correct information is cached.

Closes #11.
Pass the current window as a parameter to the render method

This change simplifies the code somewhat as the BufLeave, etc. autocmds
aren't required anymore.
Restructure segment rendering methods

Theme.get_segments() is now a generator which returns segment copies,
and it no longer handles segment content replacement.

Renderer.render() accepts a theme and a segments parameter in order to
override the default actions (calling self.get_theme() and
self.get_segments()). This is necessary to be able to cache themes and
segment contents somewhere else and provide the cached data to the
render method. The render method now also handles removing excluded
segments, which was previously handled in Theme.get_segments().

Finally, VimRenderer.render() caches all theme and segment data and
provides this data to the renderer for non-current windows.

Closes #12.
Update readonly/modified segments in default theme
Do a :redrawstatus after setting window statuslines

For some reason the statusline doesn't always get redrawn automatically
after setting the statusline property for a window, and running
:redrawstatus when a buffer or window is entered seems to resolve this
issue.
Move plugins to powerline directory
Remove unnecessary init file
Add PKGBUILD for Arch Linux users
Update documentation and version
Fix typo in doc filename
Automatically push docs with `make gh-pages`
Move vim plugin to the vim extension directory
Add font patcher script
Add font patcher symbol font
Update symbol code points in the default config
Add font patcher docs
Fix minor flake8 issues in font patcher