Implement full text index

[proycon]

Searching for text currently iterates through the whole text. To make quicker lookups possible, a full text index could be implemented (e.g. using suffixarrays), at the cost of (significant) space. This would be an opt-in feature, via a Config parameter.

[proycon]

tantivity would seem a good choice for such a feature. It's inspired by Lucene (but twice as fast), and has powerful capabilities. It can also handle offloading indices to disk and restrict the memory impact such a feature has. It could even be considered for #23 as well, especially where text is concerned.