selecting text before and after a card and deleting does not delete the card

[bantic]

If the editor looks like this:

some text in a markup section
[ a card is here ]
markup section number two

And the selecting starts at 'text" and ends at "number", hitting delete will only delete the selected text in the first markup section, leaving:

some
[ a card is here ]
markup section number two

This is because we don't walk non-markerable sections

see #107