default values for `get_prop`

[filchristou]

Similar to Julia's Base.get would be nice to have get_prop(mgr::AbstractMetaGraph, v, defaultvalue), get_prop(mgr::AbstractMetaGraph, n1, n2, defaultvalue) and so on.
The implementation should be super easy. Just checking with has_prop and act appropriately.
Personally I would find that useful. Other opinions ?