RimbleUtils - return URL as network name for custom networks

**Is your feature request related to a problem? Please describe.**
As noted in a comment in [PR #20](https://github.com/ConsenSys/rimble-web3-components/pull/20#pullrequestreview-233383276) we should use the URL instead of the word 'custom' for ethereum networks that don't match an ID.

**Describe the solution you'd like**
When a network ID does not match with mainnet or any testnets, use the current network URL as the network name.

**Additional context**
This would also help with internationalization as we would not need to translate each network name.