Support importing keys from Keybase

[le-git-imate]

Description of issue or feature request
Currently the user can import an existing key or generate a new one. An alternative solution to manage the keys is to allow users to import keys from the Keybase server.
P.S: This is a work in progress by le-git-imate team. There are some security concers that should be addressed in accordance with #13, #28.

Current behavior

• le-git-imate does not allow to import key from a key directory like Keybase.

Expected behavior

• Allow users to import key from Keybase.