Uses the hs and vs naming instead of hsync and vsync used by the previous pull request to break compatibility a little bit less as most boards, but unfortunately not all, are using hs and vs.

The following boards all feature a VGA port with an R2R DAC that is directly connected to the FPGA.

• DE0
• DE0-CV
• DE10-Lite
• Genesys2
• Mercury
• MiSTer
• Nexys4DDR
• RZEasyFPGAA2.2

The following commit factors out this resource into a VGAResource function and replaces any generic instance using Resource of such a VGA resource with a call to VGAResource instead. As some boards require an inverted signal for the horizontal and vertical sync pins, the VGAResource function also features an invert_sync parameter to invert the synchronization signals. This addresses issue #127.