Add parameter for home dir shortening

Closes #115.
Restructure and improve installation docs

The installation docs have been split into separate guides for OS X and
Linux, with OS-specific troubleshooting as part of the installation
guide.

Terminal emulator support tables have been added to both guides.

Closes #121.
Use hex strings for RGB colors in colorschemes

Previously you'd have to convert a hex number to an integer, this change
makes it possible to use a hex string instead which is much more useful.
Fix minor doc issues
Add hex colors for vim gradient
Add configuration option and docs for 24-bit terminal colors

Closes #81.
Update docs and create a common terminal feature support matrix
Add Solarized Dark colorscheme

Closes #124.
Add Solarized Dark shell colorscheme
Replace old line number character

themes/vim/help.json still used the line number character from
vim-powerline.
Remove percent placeholder from themes

The vim renderer now handles searching and replacing of the percent
placeholder internally so percent signs can be used directly in themes
and segments instead of the percent placeholder.

Refs #127.
Handle KeyErrors in common weather segment

If the query doesn't return valid weather information (e.g. if the
location query is invalid) it will return an empty weather segment.

Closes #128.