Restructure extension directories

Refs #74.
Update pip install URI in the docs
Remove Ubuntu Mono from fontconfig file

Refs #60.
Move common segment imports
Move system_load segment code
Add cpu_load_percent segment

This requires psutil to be installed, should be added to the docs later.
Memoize weather segment for 30 minutes
Update network_load segment to work with psutil
Fix highlighting when there are only 2 segments in theme

Here are two fixes:
- defaultdict makes it not throw KeyErrors.
- Replacing (False, False) with False makes it not throw vim.error (no color
  ctermbg=False)
Fix “default_module” theme key support

Reverts commit 78e54e0c840bb4a26e8c820b2f7327d11e313696 and moves
default_module into Segments.__init__.

Refs #3.
Move configuration files into separate directory

Refs #72.
Update docs

- Added notes about where common and extension-specific configuration is located
- Added common.paths config.json option description
- Fixed user-defined segments location found in segments section
- Fixed Themes/segments/module reference text
Fix problems in mode segment

- It was impossible to configure visual block and select block mode strings:
  JSON strings can contain neither raw control characters nor escape sequences
  for them
- It was impossible to override only some of the strings:
  missing key generates KeyError exception, not IndexError