Add a `--get-default-source` option

[dcaixinha]

Hi! First of all, thanks for pamixer, it's a great tool!

I'd like to have a --get-default-source option, that would print the name of the default source, similar to what --get-default-sink does. What do you think?

[concatime]

I was about to ask the same think. I would like --get-default-source to show which microphone is used.

[concatime]

~~One question too, how do I set the default source and sink?~~ EDIT: nvm, I discovered wpctl.

[cdemoulins]

It's a good idea but i don't have a lot of time currently. But i accept PR.

[LordTlasT]

there's already a --default-source option that achieves the same effect pamixer --default-source --get-volume

[LordTlasT]

there's already a --default-source option that achieves the same effect pamixer --default-source --get-volume