Support for mouse wheel scrolling

[ccrusius]

Searched on open and closed issues, didn't see this - it would be really nice if I could scroll using the mouse wheel. Hopefully this could be added without too much work.

[GReagle]

I can get the mouse wheel to work in dvtm by adding these two lines to config.h:

   { BUTTON4_PRESSED, { scrollback,     { "-1" }                    } },
   { BUTTON5_PRESSED, { scrollback,     { "1"  }                    } },

right after the BUTTON3_CLICKED line.