Add support for different reply types (in-channel, in-thread, ephemeral)

[kamaln7]

1. Add the option to configure karmabot to reply to karma operations:
   • in channel or in thread depending on where the user posted the message (old behavior, reply type: message)
   • always in a thread (reply type: thread)
   • as an ephemeral message visible only to the user (reply type: ephemeral)
2. Acknowledge reactji operations using ephemeral messages instead of annoying DMs

Closes #50.

[kamaln7]

Semaphore doesn't support Go 1.13 running tests locally:

  karmabot git:(different-reply-types) ✗ go test -v ./...
=== RUN   TestNew
--- PASS: TestNew (0.00s)
=== RUN   TestListen
--- PASS: TestListen (0.00s)
=== RUN   TestHandleSlackEvent
--- PASS: TestHandleSlackEvent (0.00s)
=== RUN   TestRegexes
--- PASS: TestRegexes (0.00s)
PASS
ok      github.com/kamaln7/karmabot 0.017s
?       github.com/kamaln7/karmabot/cmd/karmabot    [no test files]
?       github.com/kamaln7/karmabot/cmd/karmabotctl [no test files]
?       github.com/kamaln7/karmabot/ctlcommands [no test files]
?       github.com/kamaln7/karmabot/database    [no test files]
?       github.com/kamaln7/karmabot/munge   [no test files]
?       github.com/kamaln7/karmabot/ui  [no test files]
?       github.com/kamaln7/karmabot/ui/blankui  [no test files]
?       github.com/kamaln7/karmabot/ui/webui    [no test files]
?       github.com/kamaln7/karmabot/ui/webui/auth   [no test files]