Add Flowdock configuration option

This is missing component to the recent [PR](https://github.com/errbit/errbit/pull/504#commitcomment-3422990).

I thought that another configuration option for flowdock email notification option brings unnecessary complexity.

Probably the simplest way for users to fix default icons in Flowdock notifications is to just add "Errbit" icon on Gravatar for already existing email from `ERRBIT_EMAIL_FROM`.

What do you think?