Execute prep-tasks before figwheel start

[HausnerR]

Hey,
I had the same problem like author of this issue: #68 and found a way how to force lein-figwheel to compile java sources and add it to project.
Just slight change.
I'm not familiar with leiningen plugins, so probably someone more experienced should check if this solution is acceptable.

[bhauman]

Thanks for the PR I'll use it as a starting point to look into this more.

[HausnerR]

Sure, thanks!