[Feature Request] Allow the user to add custom invidious instance(s)

[sidewinder94]

Allow users to add custom invidious instances in the plugin settings page.

This would allow people to add their own instances, allowing them to curate the list of instances themselves.

Particularly if so me e want to host their own instances.

Additionally a way to remove instances and reset the default list would be handy for those cases where people mess with things too much.

[Dasnap]

Would be good to counter the current public instance issues.