Add options in useFile

[friedger]

Add a parameter options with fields

• encrypt: if false the file content is written to and read from gaia unencrypted,
• username: if a username is provided the file content is written encrypted with the public key of the user and the file content is read from the username's storage und decrypted with the own private key. (This needs implementation of publishing the public key or merging https://github.com/blockstack/blockstack-browser/pull/2000)