Add an option 'components_as_labels' to digraph_utils:condensation/1

The vertices of the graph created by digraph_utils:condensation/1 are
the strong components of the input graph. With huge components there
is a performance problem given the representation of digraph (which
uses ETS tables). As a work-around one can now use the option
'components_as_labels'. The vertices of the created graph will then be
given short names, and the components will be labels of the vertices.

The draw-back is that one will have to look up every vertex of the
created digraph by calling digraph:vertex/2 in order to get the strong
components of the vertices (the second element of the tuple returned
from digraph:vertex/2 is the label)

The performance problem was discovered by Kostis Sagonas and Stavros
Aronis while optimizing Dialyzer.