Allow using Regexp to match paths with options

Conversion options can now be defined using Regexps in addition to glob
syntax.

``` ruby
activate :webp do |webp|
  webp.conversion_options = {
    "icons/*.png" => {lossless: true},
    "photos/**/*.jpg" => {q: 100},
    /[0-9]/.+gif$/ => {lossy: true}
  }
end
```