raise permission error on Slack no_permission response

prevent Slack emoji with no URL from killing ls

ws fix

str and repr for Discord & Slack

flake8 cleanup

discord: comment cleanup

discord: util function-ize user stringification

improve User-Agent string, thanks @rlazarus

fix release notes

release 0.2.0 & add some notes on release process

discord: properly invalidate cache

fix discriminator delimiter

discord: xattr support

move xattr names to new constants file

slack: xattrs: handle / and pending writes