Detailed progress reporting during verification

[tomato42]

Because verifying even a modest certificate (1536 bit prime) takes good half a minute and can take more than an hour for big primes (8196 bit), it would be nice to have a progress bar or completion percentage reported

[tomato42]

after #25 (especially if used together with gmpy2), the verification of even 8192 bit primes takes few minutes (and a 2048bit one verifies in something like 1.5s), so per-certificate progress
notification is less important now (though it would still be nice for the --in option)