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)

Or maybe this is whence comes the whitespace requirement!  I understand the problem, but I think there must be a better workaround better than the nonsense I am about to write:

```perl
my %factors = ( 
    k => 1e3,
    m => 1e6,
    g => 1e9,
    t => 1e12,
);
my $freq_hz;

if ($freq =~ m/^(?<quantity>.+?)\s?(?<factor>k|kilo|m|mega|g|giga|t|tera)?(?:hz|hertz)$/i) {
    my $factor = ($+{factor}) ? $factors{lc substr($+{factor}, 0, 1)} : 1;
    $freq_hz = $+{quantity} * $factor;
} else {
    return;
}
```

[Far from guaranteed production-ready!] Making it better would also likely involve associating the next section with this so it knows how to generate the nicely labeled bits.