For custom commands, allow yakut to read a map file

[emrainey]

The file could be yaml or json or ini, but would map commands in words to command numbers. It could be automatically read in from known location like a ~/.yakut/custom_commands.yaml

---
custom_commands:
    - number: 300
       name: foo
    - number: 301 
       name: bar

This would allow users to do:

yakut cmd X foo instead of yakut cmd X 300 which is not descriptive.

These could be also checked for duplicates and limits (like predefined values).

[pavel-kirienko]

Do we want to consider the possibility of storing a JSON mapping in a string-typed register like uavcan.custom_commands directly on the node?