Feature request: setting for custom server URL(s)

[pacien]

I am using rmfakecloud to self-host my own private instance of the reMarkable
cloud. In my understanding, rmfuse should be able to connect to this server
software as is, since it is implementing the same protocol as the original one.

I however could not find any option in rmfuse to connect to a custom server
instead of the official one. It would be nice to expose this as a new setting.

Note: this might require some changes in the underlying rmcl library, because
the URLs of the servers seem to be constants in rmcl/const.py.