Add set_source_name function

[pantherale0]

Adds a basic function to set and reset source names. (replicating iControlAV5 functionality)

Source names cannot be longer than 14 characters so a ValueError is raised in the event that a new name is provided longer than 14 chars.

Also, setting the default paramter to True will reset the source name to its default value.

[crowbarz]

Hmmm, noticed on writing release notes for the HA integration that set_source_name is actually called set_input_name. Thinking that inputs are referred to everywhere else in the HA integration and aiopioneer as source, so will rename the function and commands before adding on the HA side.

[pantherale0]

Hmm, I actually thought I changed that, I remember I started with set_input_name but changed it to set_source_name. Must have forgotten to commit that change before rebasing the branch to reflect the later changes.

[pantherale0]

Ohh, I see, I changed it in https://github.com/crowbarz/aiopioneer/pull/23/commits/caaedb689d0606ef67eb4c92247ee22bedd4a200