Allow compile with native built solc & dockerized solc

[cgewecke]

• Adds ability to configure truffle to use natively installed solc and dockerized solc.
• Adds tests for these compiler variants in CI

Config API

// Native
compiler: {
  solc: "native"
}
// Docker 
compiler: {
  solc: "0.4.22",   // Any published image name
  docker: true
}

Solc docker images are listed here and can be installed locally by running:

docker pull ethereum/solc:0.4.22  // Example image

Instructions for installing solc binaries vary by OS and are detailed in the Solidity docs here. CI uses the linux distribution which also seems like the simplest.

[cgewecke]

@gnidan Thanks!