Ensure that gaia lite can be started with a custom / zero-height root-of-trust

[cwgoes]

If you start gaia lite and connect to an existing chain with many blocks, you definitely want to be able to verify proofs for blocks which were committed before you started gaia lite. Probably this should require confirmation by the user of some sort, and include an easy way to input a root-of-trust (as a JSON file or something) from an external source (e.g. block explorer, signed Github commit/repo, multi-signed file, etc.)

Ref https://github.com/cosmos/cosmos-sdk/pull/2210/files#r217266705

cc @HaoyangLiu

[gamarin2]

As per discussion on SDK call, here is what we need prelaunch:

• Initialization of the rest server with a root of trust (e.g. as a JSON file in .gaiacli)
• Light-sync: Users should see the headers come in
• Account query proof verification
• Documentation for delegators (done)

Possibly related to tendermint/tendermint#1771 @ebuchman?