Add facility to simplify searching for defective or missing (gap) words.

[Philip Burns]

Suggestion from Doug Knox: add search facility to locate words with missing character transcriptions ("black dot" words), missing words ("diamond" words), missing spans of text ( "..."), and undefined punctuation (black quate words).

[Craig Berry]

Resoved by:

commit bb74b7138b0530870715e802cc6a5afad4dd5bee (HEAD -> master)
Author: Douglas Knox <[e-mail]>
Date:   Thu Mar 22 13:37:42 2018 -0500

    Add a gap search feature.

    It's been quite clunky for someone who wants to fill in some gaps to
    perform a search to find them. One had to hunt around elsewhere to find
    out which special character meant what, then copy and paste the character
    (or learn how one enters it directly in one's system), and then remember
    to change the search to "ngram search," for some technical reason that is
    likely not of interest to gap searchers.

    So add a second form to the search-form.html template with easier canned
    links to searches for gaps.