Harness generation support for C/C++ source

We currently don't support this even though a very basic template exists in `templates/linux_source_harness.cpp`. When supporting this feature for source, we could approach this as so:

1. Find the best _unit test_ function candidate that maxmimizes coverage into the fuzzing target we want to generate a harness for.
2. Extract the implementation of the test and create a [DeepState](http://github.com/trailofbits/deepstate) harness from it, or maybe just stick it inside the basic template we already have.
3. If unable to, return our basic template with enough information we can fill in.