oasis wallet import add non-interactive mode

[matevz]

Currently, the only way to import an existing account is using the interactive mode. Sometimes you poses a bunch of private keys you would like to import and examine. In this case something similar to oasis wallet create would be useful. e.g.

oasis wallet import my_imported_raw_account --algorithm secp256k1-raw --secret 0x123456890abcdef1234569abcdef
oasis wallet import my_imported_mnemonic_account --algorithm secp256k1-bip44 --number 2 --secret "test test test test test test test test test junk"

The secret flag could also be omitted perhaps oasis wallet import <name> [secret] [flags]?

Related #120