Better handling of markdown extensions

[Leonardoes22]

It would be nice to have the possibility of selecting which markdown extensions we want the plugin to consider.
Currently I need to override the default arguments for each template like this -f ${fromFormat}+hard_line_breaks.

Ideally we would be able to define a variable e.g. MARKDOWN_EXTENSIONS=+lists_without_preceding_blankline+hard_line_breaks...
This variable would be taken into account by default in every template, changing -f ${fromFormat} to -f ${fromFormat}+MARKDOWN_EXTENSIONS.

[mokeyish]

This is a good idea, you can try submitting a PR.

[Leonardoes22]

Nice! When I have some free time I'll try submiting something.