resolves #878 use glyphs for checkboxes when not using font icons
Merge pull request #893 from mojavelinux/issue-878

resolves #878 use glyphs for checkboxes when not using font icons
use $~[0] instead of $& for Opal compatibility
add support for resolving includes under Opal
Merge pull request #895 from mojavelinux/asciidoctorjs-includes

add support for resolving includes under Opal
resolves #804 rewrite extensions to support extension instances

- class names, class instances and blocks are supported for every kind of extension
- also add -r flag to cli for loading custom libraries
- add convenience methods on extesion processor for creating various nodes
- ...
support lamba as process method
named group registration, document register, move all DSL methods to DSL classes
allow extension group or extensions registry to be passed to document

- includes other cleanups within Document
rename name dsl method to named; add helper method to parse content
refactor extension in Rakefile to work with new extension API
document extension APIs, use methods to retrieve all extensions
add TODO