Accept account mnemonic in config

[NicolasMahe]

The account in config should accept a mnemonic to use to generate the account in the keybase.

The logic should be:

• if config.Account.Mnemonic is not empty, generate account using config.Account info (as it is deterministic, it will regenerate the same account all the time). return account.
• if account doesn't exist based on the account.name, generate one using a generate mnemonic. return account.
• if account exist locally, just return it.

Related to comment #1459 (comment)