Added new option 'insert_links' + various fixes, tests
* Added a new option 'insert_links', which controls
whether to insert a link to the acronym's definition,
when replacing an acronym.
The default behaviour, when the option is unspecified,
is 'true', i.e., the same as previously: links are
automatically inserted.
An important use-case for this option is when the
List of Acronyms itself is disabled: links would
not work.
* Refactored the way tests are launched by 'testthat',
such that a better, more readable, report is printed.
* Added the 'wrap=preserve' Pandoc argument to the
tests, which makes Pandoc keep the line breaks as-is
from the input file. This makes it easier to write
expected files in the tests, since we only have to
focus on the content itself, and not worry about
where we should insert a line break to be consistent
with Pandoc.
* Changed the return value of the 'add_filter()'
function in the R code, from a vector of 2 strings
('--lua-filter', '/path/to/parse-acronyms.lua')
to a single string
('--lua-filter=path/to/parse-acronyms.lua').
This allows adding other Pandoc arguments in the YAML
metadata, by using a list for example.
* Improved documentation for 'add_filter()',
please see the help for more details
('?acronymsdown::add_filter' in a R console).
* Fixed a bug in the error handling of a non-existing
acronym key, where a nil value was written to
stderr.
