Add support/tests for Redcarpet 3.0.0 features
- :underline
- :highlight
- :disable_indented_code_blocks

Fix support for
- :link_attributes
- :filter_html

separate Redcarpet extensions from renderer options, add specs for missing renderer options

add specs for the missing/new redcarpet extensions, [ :lax_spacing, :underline, :highlight, :quote, :disable_indented_code_blocks, :footnotes ]

fix link_attributes option for Redcarpet

patch Tilt's reverse option aliases (they were there for RC1)

adjust test data for Redcarpet toc_data to match the current way it renders in 3.0

revert tests that aren't in redcarpet 3.0.0

add a cucumber exempt tag for mri-1.8.x

move redcarpet 3 specific extensions to a separate scenario filtered by the no18 tag