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.
Suppose the user specifies an `analyzer` and some fields in `field_analyzers`. What should be the default analyzer associated to the unspecified fields in `field_analyzers`? Should it be the default for the field or the one specified by `analyzer`? Maybe @clintongormley has some ideas.