Term Vectors API: Computes term vectors on the fly if not stored in the ...

...index.

Adds an option called `generate` to the term vectors and multi-termvectors API,
which generates the term vectors for some chosen fields, if they have not been
explicitely stored in the index.

Relates to #5184
Maybe this should actually check for `fieldMapper instanceof StringFieldMapper`. I don't think term vectors make sense on other mappers?