Improve relayer cli (#384)

- Separate the concept of "chains" and "light clients" (see docs on
`LightClient` and `Chain` for more information)
- The line between what config params are "chain" vs "light client" is
somewhat blurry, especially for the evm; comments and criticism welcome
- This may not be the best abstraction for the long run (especially with
#301), but it works quite nicely with the current architecture
- Overhaul the cli and introduce a config file format

Also addresses https://github.com/unionlabs/union/issues/328