v0.11 New custom output format, more tests
The main addition of this new version is the 'acronymsdown::md_format'
output format, which is based on 'rmarkdown::md_document'.
This format includes a few default pandoc parameters to help us run
automated tests.
In particular, it allows specifying parameters programmatically (at
runtime), rather than hardcoding them into the Rmd files.
This is particularly useful for the '--atx-headers' parameter, which
ensures that we obtain the same output on older versions of pandoc
(pre 2.11.2), but was deprecated and generates warnings if we include
it by default.
This custom format allowed us to test acronymsdown on more versions
of pandoc (we were limited to >= 2.11.2 previously).
