Enhancement: Supported multi-monitor screenshot.

[SwaggyMacro]

It doesn't support multi-screen when I screenshot, So I make this.
I don't know if this has some conflicts with the project.
The new calling method of ScreenShot is below this:

screenCapturer = new ScreenCapture();
screenCapturer.SnapCompleted += ScreenCapturer_SnapCompleted;
screenCapturer.SnapCanceled += ScreenCapturer_SnapCanceled;
screenCapturer.Capture();

[SwaggyMacro]

#19