Added field score boosting #72

[mikegoatly]

Breaking changes: IndexedFieldDetails has additional ScoreBoost property

This implements field score boosting, statically configured at index definition time:

var index = new FullTextIndexBuilder<string>()
    .WithObjectTokenization<TestObject3>(
        o => o.WithKey(i => i.Id)
            .WithField("Text", i => i.Text, scoreBoost: 10D)
            .WithField("MultiText", i => i.MultiText))
    .Build();