Allow the user to customize which line highlight is painted the first.

[keyoffecka]

This is useful when line highlights are used to show breakpoints. For example, if there is a breakpoint at line X, the line may have a specific background, now if the user moves the cursor at that line should we paint first the current line background or the background of the breakpoint line? Setting true will paint current line background and then the breakpoint line background.

[coveralls]

Coverage increased (+0.29%) to 25.51% when pulling c5301bc on keyoffecka:customizable-order-of-line-highlights into be0d5ec on bobbylight:master.