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)

Poking around, I came to realize that this requires them to be separate words, so it will match on '60 hz' but not '60Hz'. I know regex triggers are not preferred.  Is there a good solution for this, @moollaza?