New styles: fold margin checker color and edge line

styles.fold.hi
The color to use as the alternate background for the fold margin, allowing a checker effect when combined with styles.fold.bg

styles.edge
A style for inserting an edge line or background.

e.g., this creates a white line at 80 columns:
````
styles.edge = {
  mode = wxstc.wxSTC_EDGE_LINE,
  col = 80,
  fg = { 255, 255, 255 }
}
````