Add Channel Name support

[weefatboi]

This is not a bug; it's a feature request!

Youtube supports at least two formats of channel url:

1. /channel/{channel_id}, which is currently covered by the default channel_id argument in the get_channels() function
2. /c/{channel_name}, this typically pops up when Youtube channels are big enough that they get claim to an actual "name", vs a randomized ID value.

The current workaround for this is to utilize the current channel_url argument, however, thought it might prove more streamlined to support an actual channel_name argument in case users want to search videos by a list of actual channel names!

Here's an example of the additional format which this argument would support (one of my favorite streamers): https://www.youtube.com/c/Welyn

I'll make a PR with the changes; open to thoughts / comments / concerns! And, as always, thank you for your work on this useful package / contribution to the open source community!