Add a linter/format checker.

[mrice32]

I don't think there are any auto formatters out for solidity (sadly). I think we should pick a linter that will check code format and look for simple issues. solhint appears to be relatively mature, has plugins for a bunch of IDEs, and it allows customization through configs and comments.

[cc7768]

This looks great -- I like that it gives both style and security recommendations