Include description lines from feature files in a 'description_lines' at...

...tribute of Feature

My team would like to include the description from feature files as part of our test report. I've written a custom reporter for our test suite, and would like to use Spinach Feature instances to directly access the description as parsed by Gherkin. This change adds a 'description_lines' attribute to Feature, updates the AST visitor to set that attribute with the value from the Gherkin parser-generated feature node, and includes tests to cover the changes.