Allow overriding _handle_long_passwords

[bliepp]

This introduces an optional parameter for the generate_password_hash and check_password_hash member functions. It overrides the default set by BCRYPT_HANDLE_LONG_PASSWORDS. As mentioned in #45 it might introduce a bit more complexity, but there are reasonable use cases for this, e.g.:

• If you did not allow long passwords in the past and want to transition to allowing it afterwards in your already deployed application.
• If you want to support long passwords but want to reduce computational cost as most users don't use 72-byte passwords.

Both cases require the option to use both methods in the same application which was currently not possible (or at least not in a simple way as it required two instances of flask_bcrypt.Bcrypt() and manually overriding the "protected" _handle_long_passwords variable on one of those).