resolves #677 unescape escaped square closing bracket in footnote text
remove opal_ext/set.rb, to_set is now implemented in Opal
resolves #579 specialchars subs alias, shorthand subs

- add specialchars as an alias for the subs key specialcharacters
- resolve single-letter subs for inline usages such as pass:[]
resolves #685 number chapters sequentially, always
resolves #683 number sections in appendix using appendix number

- don't use normal numbering on appendix titles
- number subsections using appendix letter in first position
- use regular section numbering format when appendix-caption is blank
check context instead of type to detect sections
resolves #693 use reftext of section or block in text of xref link
resolves #695 allow spaces in reftext defined in block anchor

- also recognize only legal characters in id
strengthen anchor matches

- strengthen match for anchors
- split id and reftext in embedded section title anchor
- honor escaping of embedded section title anchor
- more anchor tests
resolves #697 honor reftext defined in embedded section title anchor

- assign reftext defined in embedded anchor to section
- register id for block even if reftext and title are empty
- don't split on comma in reftext (only split on first comma)
disallow quoted values in block and inline anchors
resolves #664 honor list continuations on indented, nested list items
resolves #300 copy custom stylesheet if linkcss is set
fix test cleanup routine in Ruby 1.8
resolves #494 output XHTML when backend is xhtml or xhtml5
Dir.home is not implemented in Opal
Requires libraries statically when running Opal engine
Remove unsupported syntax to make Asciidoctor compatible with Opal
Encoding.default_external does not exist in Opal
Add toSet method for Array class when running Opal engine