Chains Add improvements

[jackzampolin]

As a follow-on from https://github.com/cosmos/relayer/pull/49, there are a couple of features I would like to see added to relayer chains add.

• relayer chains add --url [url] - This would fetch the json and add the chain after verifying it. Think usage from a github repo (https://raw.githubusercontent.com/cosmos/relayer/tree/master/demo/ibc0.json)
• relayer chains add --cosmos [url] - This would query the /genesis endpoint to fetch most of the data and then would rely on sane defaults for gas and gasPrices based on that. This is basically figuring out how to unmarshal gaia based config.

[cwgoes]

Closing per https://github.com/iqlusioninc/relayer/issues/35.