[Bleve] Allow indexing a numeric value as text

Currently, we don't offer users any way to index a numeric value as text. But this would be very useful for many cases. Consider a JSON document containing numeric values that are identifiers. Users would never want to run a range search (though they might do so as a workaround). Instead users would prefer to do an exact match. The complication is that numbers don't have a canonical text representation. Would you want to search for `0.0000000314` or `3.14e-7`. I think we could offer a reasonable default, that would probably work well for integers out of the box, and possibly add a new optional format string to the field mapping. If it is present, in circumstances like this it would be used.