[slack_complete.pl] needs some fixes for Gitter

https://github.com/irssi/scripts.irssi.org/blob/master/scripts/slack_complete.pl

It looks like the script doesn't work with Gitter anymore.

They changed the API for https://irc.gitter.im/ and Gitter only submits the first 31 nicks for each channel's memberlist (gitterHQ/irc-bridge#52 or gitterHQ/irc-bridge#55).

While the script works fine with those random 31 nicks. It doesn't with all other nicks writing public messages. Irssi auto completes those other nicks just fine the IRC way: "nick:" There might be a way to get those nicks extracted from public messages and add them manually to the nicklist via https://github.com/shabble/irssi-docs/wiki/Channel#nick_insert_%24nick%2C_%24op%2C_%24voice%2C_%24send_massjoin

Then all nick related Gitter channels would work transparently, by manually trickle in recent nicks.