Introduce config variable indent-guide-show-beyond-column

Hi!
Great mode, very useful!

I thought it would look nicer to not show the guide all the time for the normal case, i.e. on the base line / leftmost column, so I added a config variable for that.

For code review: I wasn't 100% sure whether to include this line in the (when) conditional

    (remove-overlays (point) (point) 'category 'indent-guide)

Please have a look to see if that looks correct.