resolves #720 fixed broken passthroughs caused by source highlighting

- move passthrough placeholder characters and regexps to constant
resolves #715 replace single quote at end of word
Merge pull request #723 from mojavelinux/issue-720

resolves #720 fixed broken passthroughs caused by source highlighting
Merge pull request #716 from mojavelinux/issue-715

resolves #715 replace single quote at end of word
fix rdoc for Document#apply_attribute_value_subs()
Merge pull request #719 from Mogztter/opal-absref

Move lock_in_subs to Substituters module
Merge pull request #711 from Mogztter/opal-encoding

Encoding.default_external does not exist in Opal
changes related to #679

- remove use of mutable string operations: chop!, lstrip! and rstrip!
- calculate time elapsed using floats
- use $~ instead of numbered variables in regexp substitutions
- match endline in line break regexp
Merge pull request #726 from mojavelinux/issue-679-partial

changes related to #679
fix warnings
Merge pull request #710 from Mogztter/opal-dirhome

Dir.home is not implemented in Opal
Merge pull request #712 from Mogztter/opal-default

Rename default to defaultValue for Opal
Merge pull request #708 from Mogztter/opal-static-lib

Requires libraries statically when running Opal engine
refactor how libraries are required

- use autoload for loading stdlib classes
- remove thread safety warning in Helpers#require_library
- add LIB_PATH constant
- remove require statements for rbx in test suite
- use absolute constant path to non-Asciidoctor classes
Merge pull request #728 from mojavelinux/require-refactor

refactor how libraries are required
update Helpers#encode_uri for Opal, add tests
rename Substituters to Substitutors
add instructions for writing and executing tests
formatting cleanups

- change $> command prefix to $
- clean out trailing whitespace
- cross link to new testing section
update lis of supported Ruby versions in README