Add support for vendor commands.

[gkoh]

Originally mentioned by @gizmomelb here:
#11 (comment)_

Vendor commands, as far as I understand, are not strictly defined.
A single vendor command appears limited to 14 bytes, which helps.

However, I haven't seen enough of them to know how to handle them in a configurable way.
Experience with other embedded protocols suggests the following:

• it is likely we will need to reverse engineer the contents of the command
• commands may:
  • expect a specific response
  • expect no response

The first cut should just support hardcoded commands and responses ie.:

• if receive
• reply OR
• do something else?

Once we get here, we should be able to gain more data to make better informed design choices.