Adding full-desktop capture feature

As per my suggestion in [Issue #1](https://github.com/aholmes/Simple-screencap/issues/1), I added the ability to take a screenshot of the entire desktop consisting of more than one display. This feature uses `SystemInformation.VirtualScreen` properties. 

Rather than carelessly adding this feature I decided to also implement several options as command line arguments. 

* `-noWindow`
* `-jpeg` / `-jpg` / `-png` / `-bmp` / `-tiff`
* `-window` / `-screen` / `-desktop`

**User Interface**

`MainWindow.xaml` also received improvements. Since this application is primarily set up to be used as a headless shortcut it's not highly likely that this will be used for other than experimentation and/or testing.