Add Support for compilation of Viper contracts

### What was wrong?

Currently populus only supports solidity contracts, but many of the people to which a python framework appeals will probably be attracted to the Viper language as well. Development on viper is maturing and it would be nice to have the testing tools and other convenience provided by populus.

What would be the steps to get populus to deploy Viper contracts? I haven't given it a deep dive but I imagine we could add a flag `--viper` to the populus compile command to call the viper compiler instead? Thoughts?

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