Color total warning count depending on number of warnings

If there are no warnings, the output will be green, other wise it will
be green.

Fixes #287
I personally prefer using the longer form but for style issues I will go with the existing project style and what the maintainers prefer.

Since we talked about including Rubocop in another issue I just went ahead and looked at the Ruby Style Guide (which is what Rubocop uses) and I saw this: https://github.com/bbatsov/ruby-style-guide#ternary-operator and https://github.com/bbatsov/ruby-style-guide#use-if-case-returns.

So if even if we don't decide to change it, as per the second link I should do:
```
colour =
  if has_smells?
    WarningsColor
  else
    NoWarningsColor
  end
```
But at this point might as well change it to @oliverklee suggestion.