Add pin code change feature

[flyskywhy]

Is your feature request related to a problem? Please describe.
pin code can be created by textile init --pin and used by textile daemon --pin, but can't be changed now. Even try textile migrate -p ANY_CODE said Repo was successfully migrated, the pin code is not changed. For now, it seems the only way is to rm ~/.textile -fr then textile init --pin and sync the data form other running peer.

Describe the solution you'd like
Add a new textile command, or just add flag --pin-pre and --pin-now to textile migrate.