Add +om-next option to generate basic Om Next application

[j1mr10rd4n]

Om Next is David Nolen's next iteration of the Om cljs react interface.

The current Om Next setup guide is a bit of a convoluted leiningen/figwheel affair that's a bit distracting from the really interesting stuff. This PR would mean that could be replaced by a simple:

lein new tenzing om-tutorial +om-next

[martinklepsch]

This is great! Thanks! Makes me think we could add a bunch more things as long as there is the same 1 option/1file correspondence and you only ever get to choose one

[martinklepsch]

Merged via https://github.com/martinklepsch/tenzing/commit/34bab541ffc10bcdf58ad2a8ddedf359f5314de6

Also tried to generalize the exclusive opts assertion a bit: https://github.com/martinklepsch/tenzing/commit/ce2a30be17f98418fa97d99ca4ffbffd4f2cfe0a

[j1mr10rd4n]

Thanks for the quick merge and release. I can submit the work to update the Om Next Quick Start guide now.