Add proximity search

[tcouch]

closes #299

As described in the issue, this introduces the ability to carry out a proximity search where a search string like:

Tarquinium ~5 ferens

Would match:

Tarquinium Superbum regem adiit nouem libros ferens

In the example above ~5 represents exactly five intervening words. You can also specify upper and lower bounds for the number of intervening words:

• Tarquinium ~:5 libros matches because :5 means up to five words
• Tarquinium ~3:5 libros matches because 3:5 means between three and five words, but Tarquinium ~3:5 adiit would not match because there are only 2 intervening words

[acholyn]

Seems to work for whole words nicely. The issue mentions also working with incomplete words eg Marc ~1 varro should still return Marcus Varro but it doesn't return anything currently

[acholyn]

Would be good to update the helptext to reflect that the ambiguity is possible but you need to add the wildcard character (I didn't realise this)

[tcouch]

@acholyn I've updated the help text as well as fixing another issue which was stopping search snippets from working. Do you want to have another look before I merge?