Think through plotfreqresp()

[mchitre]

• freqresp() has been renamed to plotfreqresp() as DSP.jl just introduced a freqresp function that conflicts. Should we keep this name or is there a better alternative?
• We should default to about 30 dB dynamic range for the magnitude plot.
• We currently pass options to both phase and magnitude subplots. While this makes sense for some options (e.g. colors, line types, x-limits), it doesn't make sense for others (e.g. y-limits). How should we fix this?
• Should we allow just phase or just magnitude plot?