resolves #778, rewrite converter API; resolves #638, integrate thread_safe gem

- rewrite converter API
- separate built-in converters from template converter
- rename renderer/render to converter/convert
- make converter an extension point (resolves #778)
- base built-in converters on the converter API
- rename template_name property to node_name on AbstractNode
- make block_ prefix on file name of block-level templates optional
- use thread_safe gem for template and converter caches (resolves #638)
- introduce Stylesheets API to manage stylesheets
- move file write logic to Document
- delegate file write logic to converter that implements Writer
- remove compact logic, deprecate related options
- duplicate options passed to APIs, add test
- assign doctype / backend attributes correctly when document is loaded, add tests
- report proper error if nil is passed to load_file and convert_file
- use span tag to group kbd combination in html5 backend
- setup toc in preamble if toc attribute is preamble
- Opal compatibility fixes, use built-in HTML5 converter
- make the outline method accessible to all html converters
- document the converter APIs along with some minor cleanups in terminology
- load stylesheets from data directory
- rename ruler block to thematic_break
- add inline? and block? query methods to AbstractNode
- fix cucumber tests
- upgrade tilt dependency to 2.0.0
- minor optimizations