cmdutils: add -layouts option.

Extract of the output:

| Individual channels:
| NAME        DESCRIPTION
| FL          front left
| FR          front right
| <snip>
| SDR         surround direct right
|
| Standard channel layouts:
| NAME        DECOMPOSITION
| mono        FC
| stereo      FL+FR
| <snip>
| octagonal   FL+FR+FC+BL+BR+BC+SL+SR
| downmix     DL+DR