2.15.4

Fixes:

• In parsers.specialchar, only include special characters of enabled syntax extensions. (#150, #152)
• Avoid nesting \markdownIfOption. (#151)
• Make the \markdownSetup and \markdownSetupSnippet commands accept \par tokens. (#130)

Deprecation:

• Deprecate TeX Live 2019. (5c861e3)

Refactoring:

• Replace xstring with l3str. (contributed by @drehak, #96, #153)
• Replace keyval with l3keys (contributed by @drehak, #96, #155, #157)

Continuous Integration:

• Add Luacheck. (#154)