Add topic selection to receive notifications

[emiliogq]

Dear,

First of all, thanks for creating this set of integrations and make them open source.

I've just used the Trello bot for a small group which it uses the topic feature.

Now, when a board is synced with the Trello bot, we are asked to select a group. If we select a group with topics the bot sends the notification through the default topic (I assume), that is, "general" topic.

If the group has topic feature enabled, another select for topics would be wonderful.

Thanks in advance

[emiliogq]

Moved to integram-org/trello#4