Generate .itermcolors file when calling wal.

[SxC97]

This way the user can point iTerm2 to the file generated and every new terminal window generated will have the new theme without having to wait for the shell to load. The file will be regenerated when wal is called, thus keeping iterm's theme up to date. Taking a cursory glance at .itermcolors files, it looks like just an xml file so it shouldn't be too difficult to generate.