add new graph builder strategy method name pattern for rgraphum extension

change to be able to call Rgraphum::Graph.build_#{options[:format]} via Rgraphum::Graph.load method.

before:
    "build_graph_from_#{options[:format]}".to_sym
after:
    "build_graph_from_#{options[:format]}".to_sym
    "build_#{options[:format]}".to_sym