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)

If you're interested in that stuff, this is where the magic happens: https://github.com/duckduckgo/duckduckgo/blob/master/lib/DDG/Request.pm#L181