* 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
This stops the ui being blocked while the resource is opened. The wait
ensures that resources are reclaimed when the process finishes while
aerc is still running.
