Support multiple / custom blockchain explorer

[iancoleman]

Insight is the block explorer used for discovery and transaction building.

The insight homepage lists several different installations

insight.bitpay.com chain.localbitcoins.com search.bitaccess.ca

and it's possible for a user to run their own instance of insight.

There should be an option to specify which blockchain explorer to use.

It should include the above preconfigured inputs, but also allow for fully customized urls (eg https://www.my-insight-server.com/api).