Figure out right user-facing API for HD keys

[cwgoes]

The keybase now uses HD-derived keys by default, so we need to choose a derivation path (which may or may not be the same as the fundraiser derivation path?), support deriving multiple addresses, etc.

gaiacli add --ledger already supports some HD functionality; we can decide if we want to mirror that for normal keys or do something else.

[ValarDragon]

We should have the default HD path be clearly documented. Was the fundraiser using hardened HD derivation? If not, then I think we should use a different HD derivation path as the default.

How's the following for the UI:

$ gaiacli keys add --use-hd-path <hd path>
<if path matches a saved HD-wallet path>
Do you want to use one of the following known hd wallet seeds (enter one of the names, or no):
<Saved hd wallet seeds>
if yes ask password for that seed and complete (involves setting password for new key)
if no, ask for hd wallet seed, and complete (involves setting password for new key)

We should provide a seperate command for going from secret to master seed, and master seed to wallet seed at the given path.

Linking the graphic w/ the terms on it here

[cwgoes]

Do we still want to make changes on this front (cc @ebuchman)?

[jackzampolin]

Going to close this issue. Don't think we need to do anymore here.