Changed password-generation method for DB and secret key

The date-method you've used it okay for unique passwords but they're not random, but a truly good passwort has to be. Therefore i propose this PR which uses the systems random pool to generate the same 32 char long passwords - but now they're not just unique but also random. Also as an additional password enhancement i've added the underline to the possible password (harder to crack).

I've successfully tested this new method on Debian and macOS Sierra. This should work on almost all Linux/Unix systems.

allow user to customize renderPassword method
will help "Other password rules?" issue
lesspass/lesspass#37

allow user to customize iterations and key length for encryptLogin method.
encryptLogin use pbkdf2 with a default 8192 iterations and 32 key length.

make pbkdf2 rounds a per-site password profile option
lesspass/lesspass#38