Add visual to Frequency Spectrum goodie

Re #300, adds visual (inline SVG) for electromagnetic and/or audible frequency ranges.
![image](https://cloud.githubusercontent.com/assets/726198/3488558/cb77b122-04ef-11e4-9899-26da233c6315.png)

I think I like `$emMatch->{'subspectrum'}` over `$$emMatch{'subspectrum'}`. Any opinion on this @mwmiller?