colors: Use template files and un-hard-code export formats.

[dylanaraps]

This PR makes wal export colors using template files instead of hard-coded functions. Adding a new export format to wal will be as easy as adding a new file to the template directory.

• Extends colors: Store colors in a json format. #16
• Closes Move to template files for colors #17

TODO:

• Add background, foreground and cursor to all templates.
• Add templates for all existing formats.
• Add back printing of export: Exported var colors.
• Add back xrdb -merge of x colors.
• Convert putty colors to rgb.
• Fix tests.