Support multiple compiler versions

[mikeshultz]

Add support for multiple compiler versions. Requirements:

• Add ability for users to seamlessly switch between versions
• Include downloader
• Kind of mimic nvm experience
• Informed by pragma? Should contracts be able to be compiled with different versions?

Tentatively scheduled for v1.1.

[mikeshultz]

Added SOLC_PATH so users can specify a specific compiler. Doesn't solve this issue, but helps for now.

Since projects tend to use a single copmiler version across all contracts, it might be good to have the ability to specify a compiler project-wide(networks.yaml, or run a command to set in metafile.json?).