Added support for personalization vector for dangling nodes

Hi,
Here is my attempt at adding the support for personalization vectors for dangling nodes. The default behaviour is implemented via R default function flags, although there is also some #ifdef code in prpack.cpp to provide a copy of the reset vector if the resetdangling vector is absent.

Testing is not yet thorough, but everything compiles and installs, checks pass and documentation updated.

Thanks for your help, and I hope this patch makes life easier for you.
@gaborcsardi can probably answer it better but I think that the R interface generator automatically replaces underscores with dots in parameter names. For instance, the specification of `igraph_contract_vertices` simply lists the `vertex_attr_comb` attribute as is, and the R interface magically receives `vertex.attr.comb`.