location cache index needs to be invalidated after actions that change a document #91

PPI leaves it up to the user to flush locations after adding or removing things

Please note that this index should always be cleared using C<flush_locations>
once you are finished with the locations. If content is added to or removed
from the file, these indexed locations will be B<wrong>.

and of course it does not document that on insert_before/after

Probably the easiest fix is to add a flag to the document that indicates it needs to revalidate locations.