rvn_download enhancement - use local exe if already compiled

[rchlumsk]

For macOS and linux users, would be helpful to have an option to place a copy of Raven.exe from a specified path (e.g. where it is normally compiled on one's system) to the same RavenR location that it would otherwise be downloaded to. This would skip the download nad instead just copy the exe to the specified path, and otherwise the rvn_run function can be used as normal.

The rvn_run can be used when the exe is manually copied over, but this is less fun than letting the script handle finding the correct location

[rchlumsk]

Done with recent push - see copy_path parameter in rvn_download function