More Like This Query: Adds the ability to specify the analyzer used for each Field

When using multiple items, the user may want to specify which analyzer to use
for each field. Previously, either the analyzer specified by 'analyzer' would
be used for all the fields, or if not set, the analyzer associated with the
field would be chosen. This commit provides the ability to fine grain which
analyzer should be used for each field by providing a new 'fields_analyzer'
parameter to the More Like This Query.
if we pass the var `analyzer` to `addMoreLikeThis` instead of `DefaultAnalyzer` then it is almost right, only that I wanted to have that if `field_analyzers` is specified (even if empty) then the default analyzer is always overridden by the one associated with the field.