Add options to URI::GID to store simple metadata as query parameters.

Fixes #39.

Ready for review :grin:
All but the last value are swallowed.

```ruby
s = URI::GID.build(..., params: { multi: %w(one two) }).to_s # => "...?multi=one&multi=two"
URI::GID.parse(s).params[:multi] # => 'two'
```