Add support for context URL option

[kd-zero]

Summary

• add context: support to Fulfil::Client#search and Fulfil::Client#count
• serialize hash context values to JSON and append them as URL query params
• pass context: through Fulfil::Model#search and Fulfil::Model#count
• add client tests for context query support
• document the option in README and changelog

Closes #63

Testing

• bundle exec ruby -Itest test/fulfil/client_test.rb test/fulfil/model_test.rb
• bundle exec rubocop lib/fulfil/client.rb lib/fulfil/model.rb test/fulfil/client_test.rb