Usage from inside project subdirectories

`cppargo` fails to run when in a subdirectory of a `cppargo` project. This 
could be fixed by utilizing some sort of `.toml` file which marks the root of 
the project such that `cppargo` can search for the `src` and `target` 
subdirectories for correct compilation and running no matter where inside the 
project `cppargo` is run.
