Config::load supports at: path option

When used new configuration will be merged starting at path indicated by this option. E.g.:

    config.load 'defaults.cson'
    # a:
    #   a1: 1
    #   a2: 2
    # b:
    #   b1: 3

    config.load 'b.cson', at: '/b'
    # b1: 4
    # b2: 5

    config.get '/'
    # a:
    #   a1: 1
    #   a2: 2
    # b:
    #   b1: 4
    #   b2: 5
0.3.0