Add options to colorize host

[Safranil]

C.f. issue #381.

This PR contain the code to add the fg and bg to Powerline. It take an RGB color in hexadecimal format.

Example .powerline-shell.json file:

{
  "segments": [
    "...",
    "hostname",
    "..."
  ],
  "hostname": {
    "colorize": true,
    "fg": "000000",
    "bg": "ffffff"
  }
}

[rdollinger]

Great solution, finally I can read the hostname