render config with confp

[fipwmaqzufheoxq92ebc]

Fixes #209

This PR adds the possibility to preprocess the config-file with confp by using a new (optional) command line argument
--render which should point to confp config file. The mqtt-io config file is used as template.

For that to work I made small changes to confp ( flyte/confp#8 )

This PR is a draft, as confp must be added to the requirements, but I have to wait for a new release with the changes of flyte/confp#8

[flyte]

I've made a release incorporating your changes on confp, which is now at 0.4.0.

I like this solution, it's nice and neat. I'll need to spend some time going through it and writing some docs for it, but happy to merge it before doing that.

At some point I'd like to have it render the config file by default, but this gives us a way to test it out nicely in the mean time.