Add '--commands' command line argument to restrict input to a limited set of commands

[ideasman42]

In situations where only a limited set of commands is needed, it would be useful to pass this list in as an argument.

This has the advantage that dictation could end immediately once a unique command was matched.

It would also allow for fuzzy matching if exact matches could not be found.

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Example:

COMMAND="$(nerd-dictation begin --commands=valid_commands.txt --timeout=1.0)"