Add support for Xapian::QueryParser::set_max_wildcard_expansion()

[glaville]

When a Xapian database contains many variants of the same, short words (such as test1, test2, test3, test4, test5, test6...), researching wildcard patterns such as "test_" becomes very expansive, both in time and memory, than longer one such as "test4_" or "test40*", for example.

The raw Xapian binding provides a QueryParser::set_max_wildcard_expansion function [1] which allow the user to limit the number of expansions retained in this use case (by default 0, or unlimited): However, there does not appear to any easy way to achieve using the Xapian-fu.

Would it be possible to add support for a :max_wildcard_expansion option in search requests?

[1] http://xapian.org/docs/apidoc/html/classXapian_1_1QueryParser.html#7651d48cdc661c0605c475925170cc71