`_solidity.py` functions should support general `**kwargs`

[konradkonrad]

Otherwise it is not possible to adjust compiler settings (i.e. --optimize) when calling Solc.* classmethods, or when using tester.state.abi_contract(...).

Furthermore, there should be a (generic?) way to use currently unsupported flags, such as

--compiler-runs <n>

(see solc --help for all the options/flags).