More granular compile targets

[elenadimitrova]

When working with a solution that uses a mix of solc versions for contract compilation there needs to be a better granularity when defining compilation targets. Currently etherlime compile takes the contracts folder by default. A different folder can be targeted using the --workingDirectory option. What will make this fully flexible is allowing single contract targets to be compiled and also single contracts excluded from compilation of a target folder.
This is very useful when working with migrations between solc releases as rarely can a solution be upgraded all at once. Normally in gradual migrations we prefer to selectively upgrade contracts without disturbing the contracts folder structure.
As an example, the argent-wallet solution has the following contracts structure and respective solc versions

/ contracts
    /infrastructure - solc 0.5.4 except for `WalletFactory` and `TokenPriceProvider` contracts
    /legacy              - solc 0.5.4 
    /modules          - solc 0.6.8 
    /wallet               - solc 0.6.8

[ochikov]

You can pass to the optional param: workingDirectory a path to a single contract.
However, we should add a single exclusion of a file.