If a search pattern that looks like an email address is found
an additional query using Web Key Directory will be performed.
Implements basic flow described in "Key Discovery" [0] I-D.
Querying SRV records is not supported.
Fixes partially #2270.
[0]: https://tools.ietf.org/html/draft-koch-openpgp-webkey-service-05#section-3.1
GitHub allows dash / minus characters in usernames (e.g. "wiktor-k" or
"open-keychain"). This change extends the regular expression to capture
this missing character.