don’t generate STORY_INDEX if a conflicting story exists

This is a fix for the dreaded

    ERROR: Two different tasks can't have a common target.
    'output/index.html' is a target for render_pages:output/index.html and
    render_indexes:output/index.html`.

This works by checking pages that would appear in the index, and if
one of them has a matching destination (i.e. slug == index), the story
index is not generated.  Most users are likely to want it to work
this way.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Merge pull request #1356 from getnikola/fix-story_index

don’t generate STORY_INDEX if a conflicting story exists