Arbitrary static document feature support

[lgrz]

Some static document features are best computed by other software of a specific hand rolled program. For example, some static web features such as PageRank, Inlink/Outlink count are usually best built from other software.

Supporting a way to import these features in a common format would be desirable. One possible approach is to follow what Indri does with how it imports document priors with make_prior.