Defining conversion options in Middleman config.rb

Related to #2.

Options supported by cwebp and gif2webp command line tools are now
defined in config.rb with following manner:

``` ruby
activate :webp do |webp|
  webp.conversion_options = {
    "icons/*.png" => {lossless: true},
    "photos/**/*.jpg" => {q: 100}
  }
end
```

Hash key in conversion_options is glob pointing to cwebp options
hash. Those options will be used to files matching the given glob
pattern.