Add google normalized distance and chi square to significant terms

For google normalized distance, see
"The Google Similarity Distance", Cilibrasi and Vitanyi, 2007
http://arxiv.org/pdf/cs/0412098v3.pdf

For chi square, see
"Information Retrieval", Manning et al., Eq. 13.19
ParseField.match should throw an exception if the flag contains strict and a deprecated name was used. It does currently not make sense at all as the flag is always empty. The reason for this is that the parse flags are currently not passed on to the different aggregation parsers. 

I added this in https://github.com/elasticsearch/elasticsearch/pull/6561 and remember I had a discussion about it (but could not find the comment anymore). The idea was to have it in place  and once the flags are properly passed, we can use it properly. It is also done this way in [Aggregator](https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/search/aggregations/Aggregator.java#L105)

However, I now think adding the check with empty flags was a bad idea as it causes confusion already. 
Should I remove this check everywhere?