Expose wkd-hash generation as a standalone function

[pravi]

This can help in reusing this logic in cases like
mailvelope/keyserver#121

[pravi]

For now I have copied the code here mailvelope/keyserver#146 but sharing maintenance via a wkd-node module would be better.

[pravi]

For example gpg-wks-client already exposes these options, it'd be nice to have this directly in javascript as well.

$ gpg-wks-client --print-wkd-hash bernhard.reiter@intevation.de
it5sewh54rxz33fwmr8u6dy4bbz8itz4 bernhard.reiter@intevation.de

$ gpg-wks-client --print-wkd-url bernhard.reiter@intevation.de
https://openpgpkey.intevation.de/.well-known/openpgpkey/intevation.de/hu/it5sewh54rxz33fwmr8u6dy4bbz8itz4?l=bernhard.reiter

Expose wkd url in addition to wkd hash as well.