the markup argument can be NULL in highr v0.4

[ci skip]
use title case for title
fixes #872: add the table environment when caption is specified
export engine_output() for @casallas per discussion in #866
clean up after #826: only tweak `output` when text is NULL

also consider the case output = 'foo.html' besides output = 'foo.md'
add @gdetrez to the list of contributors for #826
bump version