* Get rid of open_darwin
It just lead to code duplication for a simple one string change.
Instead we query it during initialization
* Accept user provided arguments
"open" on MacOS accepts things like -A to use a specific application
Pass trough arguments the user provided in order to facilitate this
* Refactor the function to a struct
This makes it more convenient for the caller and avoids signatures like
lib.OpenFile(nil, u.String(), nil) which are fairly unreadable