Add support for marking blocks by row, column as well
Rename: shorter function names
Add tests for new arbitrary marking
Refactor some logic into the model; document some things
Only redraw highlight canvas when marking -- should improve performance
Fix a bug, document mark(), unmark()
Rename markedBlockKey to nextMarkedBlockId
Merge pull request #20 from dabbler0/arbitrary-highlight

Add support for marking blocks by row, column as well