Add support for bcrypt_sha256

[AlbertVeli]

Feature request. Add support for this format. It is used by some web frameworks, like CTFd. Python code for the format:

>>> from passlib.hash import bcrypt_sha256
>>> bcrypt_sha256.hash('john')
'$bcrypt-sha256$2b,12$B2bcgGrxCAOZdbU/TDlP6e$8G1QvMXLUpqw1l79lmIRbFMpw1tJwqS'
>>> bcrypt_sha256.verify('john', '$bcrypt-sha256$2b,12$B2bcgGrxCAOZdbU/TDlP6e$8G1QvMXLUpqw1l79lmIRbFMpw1tJwqS')
True

The format internally uses:

bcrypt(base64(sha256_raw($plain)))