Enable custom default node ip address in config file

[gavinly]

currently, if i'm using an RPC (instead of a full node) i need to include the ip address at the end of every command, like this:
namadac epoch --node tcp://rpc.housefire.luminara.icu:80

in future, i should be able to set this ip address in a config file (ideally using a CLI command) so that this updates to become the default. then i my command would like like this:
namadac epoch

(fyi, Cosmos SDK chains work this way)

[tzemanovic]

This is a duplicate of #47, but thanks anyway!