use regex to capture cabal key instead of URL

[cblgh]

apparently the new URL() behaviour is unpredictable. chrome, node, and
firefox all return different values when a cabal key is passed in.
(searchParams always seems to work, however)

for node the cabal key can be found in uri.host, in chromium/electron
it's found at uri.hostname, for firefox both of those are empty, but
uri.pathname gives the cabal key prepended with two slashes (but that
doesn't work in node...)

until the IANA change lands / has a standardizing effect, let's just use
a regex to capture it.

this patch also changes isHypercoreKey to use the same regex as cabal-client.
i dunno if hypercore has a strict lower-case character requirement, however

fixes #88