Add encode/decode commands as helpers for i3bar block scripts:

 * Add encode command to produce an i3bar block
 * Add decode command to read properties of a click event sent by i3bar
split main.go in multiple files
README: add missing display_song func in mpd-nowplaying.sh
README: update i3cat -h
encode: add flag to output single block or array
README: add instructions for i3cat encode/decode