colors: Store colors in a json format.

[dylanaraps]

This PR changes the colorscheme file format in wal from a list of hex colors to a json file. This makes it easier for users to create their own colorscheme files for use with wal. This will also make it easier for users to use the colors in other places (their own scripts or etc), json is easily parseable.

This PR also allows the user to specify the background, foreground and cursor colors independent of the colors 0-15.

Closes #15.

TODO:

• Add support for saving wal generated schemes as json.
• Update other functions to use json.
• Update existing tests.
• Write tests
• Firgure out what these escape sequences control:
  • sequences.append(set_special(13, colors["foreground"]))
  • sequences.append(set_special(14, colors["background"]))

Example json scheme file:

{
    "special": {
        "background":"#3A5130",
        "foreground":"#FAF9F5",
        "cursor":"#FAF9F5"
    },

    "colors": {
        "color0":"#3A5130",
        "color1":"#E3A19D",
        "color2":"#E1CEAE",
        "color3":"#D6DDCC",
        "color4":"#F1D2CB",
        "color5":"#F5E9D6",
        "color6":"#F9F0E5",
        "color7":"#FAF9F5",
        "color8":"#999999",
        "color9":"#E3A19D",
        "color10":"#E1CEAE",
        "color11":"#D6DDCC",
        "color12":"#F1D2CB",
        "color13":"#F5E9D6",
        "color14":"#F9F0E5",
        "color15":"#FAF9F5"
    }
}

Note: Tests will fail since I've got a # TODO: comment.