Refactor cli functions - remove hardcoded urls

[edchapman88]

• move config.rs from trustchain-core to trustchain-cli and rename core struct to cli to contain cli config options
• cli config options to include root_event_type and resolver_url (should resolver_url be a cli arg instead?)
• remove all hardcoded url's from trustchain-cli

suggestion to do this work on #79 with open PR #80

[edchapman88]

mostly completed with: https://github.com/alan-turing-institute/trustchain/pull/80/commits/4eeb804f8cebc76d2a3d46909571a978986a14d4

only task remaining is:

• should resolver_url (now named ion_endpoint) be a cli arg as-well/instead?

[sgreenbury]

mostly completed with: 4eeb804

only task remaining is:

* should resolver_url (now named `ion_endpoint`) be a cli arg as-well/instead?

For now, retain a simple CLI API without resolver as an argument.

Closing with #80 merged.