Simplify source code input for verification

[zoeyTM]

fixes #706

[alcuadrado]

Something that we discussed on a call.

Let's add a flag includeUnderlatedContracts to the verification function that core exposes. By default it should be false. If true, we don't minify the solc input.

Then, in the plugin, we add a --include-unrelated-contracts flag to verify, which enables that flag.

If the verification fails and the flag wasn't on, then we recommend the users to retry with --include-unrelated-contracts.

[alcuadrado]

LGTM