feat: add "rounds" possibility

[furiozo-ga]

a slight modification of the salt extraction to allow verification of hashes of the form $id$rounds=yyy$salt$encrypted, as documented in the crypt(3) man page.
the encrypt method is fine as you can call it as :
crypt.encrypt('pass','rounds=99999$'+crypt.createSalt())
and it works

[jimmyolo]

@furiozo-ga LGTM, can you write a simple test & update the document (add an example should be fine)