Avoid taking an exclusive lock on Litra devices on macOS (#93)

Currently, when a Litra handle device is opened with `open()`,
on macOS we take an exclusive lock on the device, which stops
it being managed by other applications.

This isn't a problem if the device handle is quickly closed
again (e.g. if the application exits), but it can be annoying if
you have a long-running application that holds onto the device
handle for a long time (e.g. `litra auto-toggle`).

As an example, if you run `litra auto-toggle` and then try to
turn the device on or off with `litra on` or `litra off` in
another shell, it would fail with an error:

> HID error occurred: hidapi error: hid_open_path: failed to open IOHIDDevice from mach entry: (0xE00002C5) (iokit/common) exclusive access and device already open

This switches to taking a non-exclusive lock on macOS, affecting
use of this code as a Rust library and our long-running
`auto-toggle` command.

src/lib.rs

@@ -49,7 +49,10 @@ impl fmt::Debug for Litra {
 impl Litra {
     /// Initialize a new Litra context.
     pub fn new() -> DeviceResult<Self> {
-        Ok(HidApi::new().map(Litra)?)
+        let hidapi = HidApi::new()?;
+        #[cfg(target_os = "macos")]
+        hidapi.set_open_exclusive(false);
+        Ok(Litra(hidapi))
     }
 
     /// Returns an [`Iterator`] of connected devices supported by this library.
@@ -177,7 +180,7 @@ impl Device<'_> {
     }
 
     /// Opens the device and returns a [`DeviceHandle`] that can be used for getting and setting the
-    /// device status.
+    /// device status. On macOS, this will open the device in non-exclusive mode.
     pub fn open(&self, context: &Litra) -> DeviceResult<DeviceHandle> {
         let hid_device = self.device_info.open_device(context.hidapi())?;
         Ok(DeviceHandle {