Add support for node_modules imports

[SeekTheError]

Similar to what truffles allow, it would be nice to be able to import contracts that are located in the node_modules folder rather than having to copy them over inside of the contract path.

ex:

import "@appliedblockchain/trufflib/libraries/Sig.sol";

where "@appliedblockchain/trufflib" is an installed node module

[mtomov]

To handle that, I'd usually override the solc parameter, like:

  // test/helpers/init.js
  const solc = require('@appliedblockchain/cobalt/solc')({
    root: join(__dirname, '..', '..', 'contracts'),
    solc: 'solc zos-lib=../node_modules/zos-lib @appliedblockchain/trufflib=../node_modules/@appliedblockchain/trufflib',
    allowPaths: '../,'
  })

and then:

 const { web3 } = require('@appliedblockchain/cobalt/web3')({
    solc
 })