fix issue #5: indent in cases if plot[:][:]

added an optional set of range specifications to
`gnuplot-continued-commands-regexp' to work with commands like the
following:

plot[-pi:pi][-1:1] sin(x),\
                   cos(x)

plot [-pi:pi] [-1.3:1.3] [-1:1] sin(t),\
                                t**2,\
                                cos(t) * sin(t)

see tests/indentation.gnuplot