Allow whitespace before protocols.

Hi @rgrove,

Thanks for an excellent library!

I recently received a bug report on [Feedbin](https://feedbin.com) about a broken image. The source in the XML looked like:

```html
<img src="
http://img.thedailywtf.com/images/14/q4/e141/Pic-2.png" />
```

But after it was sanitized it looked like:

```html
<img>
```

It appears that any attribute that is checked for a valid protocol will be stripped if it contains leading whitespace.

Not sure if this is the right place to address this. The other idea I had was to allow for a regex when specifying allowed protocols like:

```ruby
protocols: {
  'img' => {'src' => [/\A\s*http/i]}
}
```