Allow override of screenshot file name or make it compatible with fastlane deliver

[trevoriancox]

To work with fastlane deliver, for automated uploads of screenshots to App Store Connect, the screenshot of a 12.9-inch iPad Pro (3rd generation) must contain either the string "iPad Pro (12.9-inch) (3rd generation)" or "ipadPro129". Xnapshot creates: "1 iPad-Pro--12-9-inch---3rd-generation-".

It would be nice to be able to override TakeScreenshot (or a new name method) to change the filename format.