Add import remapping documentation to `solc --help`
Add import remapping documentation to `solc --help`

Derived from the docs found [here](https://solidity.readthedocs.io/en/develop/miscellaneous.html#using-the-commandline-compiler).
Fixes #1207.

ruchevits/solidity,ruchevits/solidity,ruchevits/solidity,ruchevits/solidity