Add label argument to createChan API endpoint

[813492291816]

jsonrpclib throws errors sometimes due to the auto-generated labels that are based on the passphrases I set. Example:

(-32603, '<class 'api.APIError'>:API Error 0021: Unexpected API Failure - bad interpolation variable reference '%(793569%(4"]'')

This "%(793569%(4"]" is a substring of my label that's based on the passphrase and the error prevents any data from being received.

Because the passphrases I use are generated in a specific way, I can't just exclude certain character combinations as possibilities. Therefore, I propose adding the ability to set the label at the time of the chan creation, which will allow me to set the label to something different/safer than the passphrase.

[PeterSurda]

I think this should be fixed differently.

[813492291816]

I don't know if this is an issue with jsonrpclib or bitmessage, or even how to debug it, but with regard to this PR, I see utility for a number of use cases in allowing the user to set the label of a new chan at creation.