Allow to control showing grid rows and columns

[ao2]

The grid columns can be distracting when debugging the layout using
other tools, like for instance the svg grid from Susy:

#page {
@include set-container;
background: susy-svg-grid() no-repeat scroll;
}

So add two new configuration variables ($debug-grid-show-rows and
$debug-grid-show-columns) to control showing rows and columns of the
megatype debug grid.

The default behavior is unchanged, the user has to explicitly disable
the display of rows and columns in their configuration.

NOTE: The default output should also be identical to the current one, I am not sure if there is some more elegant way to avoid the rather ugly string concatenation I used.

Thanks,
Antonio