add graph builder strategy method name pattern: "build_#{options[:format]}"

before:
    "build_graph_from_#{options[:format]}".to_sym
after:
    "build_graph_from_#{options[:format]}".to_sym
    "build_#{options[:format]}".to_sym
Merge pull request #4 from meltedice/graph-builder-strategy-new-method-name-pattern

add new graph builder strategy method name pattern for rgraphum extension