Add highlight color option

I was missing the option for defining the highlight color when diffing the captured screenshots.

Instead of doing this:
```ruby
@config['highlight_color'] ? @config['highlight_color'] : 'blue'
```

should I instead have done it like:
```ruby
@config['highlight_color'] || 'blue'
```

Or how is that normally done in Ruby?

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Please let me know if you want me to squash the commits or anything else.