User can set a description for their custom command that shows up when executing :ls

The builtin commands have descriptions but custom ones do not.

It would be nice to be able to do something like this:
`ssh alpha.cmd.io :create ambiguous "not anymore"`
`ssh alpha.cmd.io :create ambiguous2 "much less so"`

Then the :ls command would show something like:
```
☰ Commands
ambiguous not anymore
ambiguous2 much less so
```