New filter hook: es_meta_query_meta_value
Applies a filter to meta value terms before execution.
This will allow adapters to do things like transform to lowercase if the `meta_value` backend is mapped to `raw_lc`, etc.
WPCOM VIP Adapter: Use lowercase terms for meta value queries

* Hook into 'es_meta_query_meta_value' filter enabled in 6505d48
* Map the meta value field to 'meta.%s.value.raw_lc' (instead of 'meta.%s.value.raw')

Enables case-insensitive meta_value querying.
Merge pull request #11 from jblz/master

Enable normalization of meta value terms via filter application