Add ability to specify allowed paths for solidity compiler

[dostu]

Currently, it is not possible to compile a contract which imports a contract outside of its directory.

import "../Customer.sol";

> solidity-flattener/src/solidity_flattener contracts/contract_tracking/CustomerRegistry.sol
> contracts/contract_tracking/CustomerRegistry.sol:4:1: Error: Source "contracts/Customer.sol" not found: File outside of allowed directories.

I propose we should allow the user to specify --allow-paths argument for solc.

> solidity_flattener contracts/contract_tracking/CustomerRegistry.sol --allow-paths="$(pwd)/contracts"

[izetex]

I tested and it worked for me

[dostu]

Closing in favor of #16