Add support to pass extra arguments to flatc#11

Added the ability to pass a list of arbitrary arguments to flatc so that we can use arguments other than the ones supported by the current `Args` struct. While it might be good to extend the `Args` struct to include more of the supported flatc args, I think it's still important to support any argument the user wants. Solves issues #9 Update Flatc::from_path to take &Path instead of PathBuf. Taking a PathBuf forces `from_path` to take ownership of the passed in`PathBuf` which forces the user to clone it if they want to use it again. Taking a `&Path` doesn't take ownership but still clones it internally.