Term Vectors: Support for artificial documents

This adds the ability to the Term Vector API to generate term vectors for
artifical documents, that is for documents not present in the index. Following
a similar syntax to the Percolator API, a new 'doc' parameter is used, instead
of '_id', that specifies the document of interest. The parameters '_index' and
'_type' determine the mapping and therefore analyzers to apply to each value
field.
Well this is exactly what I tried to avoid, that is to create these mappings. Do we really want to allow the user to create new mappings when providing a document where the fields are not in the original mapping? Those fields could have been provided by mistake. I wanted to just ignore these fields, and not dynamically create a mapping for them, but there does not seem to be an easy way to do so, or is there?