Allow opening links in the background

[serenium]

If Finicky is called to open a link in the background, it should pass this request onto the browser. After a quick hunt through the documentation it looks like the flag responsible for this is NSWorkspaceLaunchWithoutActivation in
NSWorkspaceLaunchOptions when calling openURLs:withAppBundleIdentifier.

EDIT: it might be nice to allow this to be changed with the API too, for example, revealed to the function in the opts variable, and possibly changed with a flag in the return value.

[johnste]

Fixed in v0.4 https://github.com/johnste/finicky/releases/tag/v0.4

Did not add the flag to the opts variable this time around, but will consider it.