Add support for kramdown options

[dometto]

For commonmarker, it is already possible to pass in parsing and rendering options in the GitHub::Markup.render call, e.g.:

GitHub::Markup.render("test.md", "hello <bad> world", options: {commonmarker_opts: [:UNSAFE]})

This PR adds support for the same feature for the kramdown implementation, allowing e.g.:

GitHub::Markup.render("# Test\n", options: {:kramdown_opts => { :auto_ids=>false}})

I've also added tests, which necessitated requiring kramdown on both the JRuby and MRI platforms (instead of just JRuby). If this is undesirable, the tests could also be left out by reverting the last commit from this PR.

[dometto]

@aharpole any chance this could be merged?

[dometto]

The test failure is unrelated. The cgi gem is apparently calling String#delete_prefix, which doesn't exist on ruby 2.4.1. May be related to 2.4's general oldness and this warning: Your RubyGems version (2.6.14.4) has a bug that prevents "required_ruby_version" from working for Bundler.