Make the file path/file name display style configurable

Closes #102.
Show shortened path in NERDTree buffer

NERDTree (and maybe other buffers in the future) now only shows the most
important parts of the current path. $HOME is shortened to "~" and if
the path consists of more than `threshold` (currently 4) directories it
will be shortened, i.e.
"/home/user/a/very/long/path/indeed" will be shortened to
"~/…/long/path/indeed".

Closes #96.
Create paste mode indicator

When paste mode is active (see :h 'paste') "PASTE" is shown as
a separate segment to the left of the mode indicator.

Closes #17.
Make it possible to provide segments without padding
Separate the file path into its own segment

By doing this (and disabling padding for the segment) the file path can
be highlighted with a less pronounced color than the file name.

Refs #102.
Update default color scheme and segments

Some colors have been tuned, and the column identifier has been reverted
to the initial display style (using ':' as the column separator instead
of a double vertical line).
Strip ./ from file paths in Pl#Functions#GetFilepath()
Make 'relative' filepaths the default value

Vim's default statusline uses relative file paths as the default so it
makes sense to have that as the default value in Powerline as well.

It's easy to reconfigure the plugin to display only the file name:

	let g:Powerline_stl_path_style = 'filename'

Refs #102.
Create functionality for overriding symbols

Closes #66.
Refs #100.
Create functionality for overriding dividers

Closes #105.
Fix errors when $HOME contains weird characters

Closes #106.
Initial commit
Add initial project files
Add vim statusline example

This is currently rendered with the terminal renderer, and is just
a simple proof-of-concept of how vim statuslines can be defined with the
Python API.
Fix soft separator issue with vim divider segments
Join the fg/bg/attr methods of the renderers

This change joins the fg/bg/attr methods into a single hl method in the
renderers. This provides the same functionality, and it simplifies the
terminal rendering by being able to join all the attributes into one
escape sequence.

It's also a necessary change for the vim renderer, as this renderer
needs to log all the highlighting and create separate highlighting
classes for every single color and attribute combination. The only way
to do this is to have a single highlighting method.
Fix default segment attribute value
Create basic vim segment renderer

This commit also includes a basic proof-of-concept demo for creating vim
statuslines. When creating vim statuslines you basically need to first
create the statusline the same way as the terminal demo, use the vim
renderer and then afterwards loop through the collected highlight groups
in the vim renderer and write those as vim highlight statements.

Vim obviously needs a ton of wrapper code to make everything work
properly (separate modes, callbacks, all that stuff) so the current demo
only shows some basic statusline highlighting.
Change wording: separator -> divider
Move vimscript hl statement generation to vim renderer