support for literal footnote matching instead of value

We had a need to input citations in completion each time, so the document would look like this

Paragraph paragraph paragraph[fn]Some long winded citation, pp 281(59):38 x.[/fn]
Paragraph paragraph paragraph[fn]Another very long citation[/fn]
Another paragraph, we just repeated the 1st citation - but it is exactly the same, literally[fn]Some long winded citation, pp 281(59):38 x.[/fn]

Since we don't specify value attributes, there is no way to link the citations together. There are 100s+ of citations entered in this way, so going back and reordering by value wasn't an option. I've attached a patch to 2.3 that makes this possible. If the previous literal footnote is found in $store_matches, we set the $value to the $value returned by _footnotes_helper_find_footnote().

Would appreciate testing and review! Here is a sample text block that I tested with:

<p>Citation 1, no value set [fn]Zuczek, DT 1[/fn]</p>
<p>Citation 1, no value set [fn]Zuczek, DT 1[/fn]</p>

<p>Citation 10 - value set [fn value="10"]Some citation here.[/fn]</p>

<p>Citation 2, no value set [fn]Zuczek, DT 2[/fn]</p>
<p>Citation 2, no value set [fn]Zuczek, DT 2[/fn]</p>
<p>Citation 3, no value set [fn]Zuczek, DT 3[/fn]</p>
<p>Citation 3, no value set [fn]Zuczek, DT 3[/fn]</p>
<p>Citation 4, no value set [fn]Zuczek, DT 4[/fn]</p>

<p>Citation 10 [fn value="10"][/fn]</p>
<p>Citation 10 [fn value="10"]Text that shouldn't be shown[/fn]</p>

<p>Citation Auto - 1 [fn]Autonumbered Citation 1[/fn]</p>
<p>Citation Auto - 2 [fn]Autonumbered Citation 2[/fn]</p>
<p>Citation Auto - 3 [fn]Autonumbered Citation 3[/fn]</p>

<p>Backreference Citation 10 [fn value="10"][/fn]</p>

<p>Backrefence Zuczek, DT 2[fn]Zuczek, DT 2[/fn]</p>

If you try this code out, the identical citations should reference one citation instead of being created individually.