Declare PANDOC_DATA_DIR and PANDOC_TEMPLATES_DIR as ENV variables

[daamien]

closes #274

[alerque]

This looks like a good improvement to me, with the caveat that it feels like this might be a breaking change for somebody downstream and a little difficult to figure out why it broke. I don't have a way to test this and haven't reasoned out exactly what could happen, this is just a gut feeling that even if the change is for the best it might leave somebody hanging that depended on some override of the previous status quo.

[daamien]

This looks like a good improvement to me, with the caveat that it feels like this might be a breaking change for somebody downstream and a little difficult to figure out why it broke. I don't have a way to test this and haven't reasoned out exactly what could happen, this is just a gut feeling that even if the change is for the best it might leave somebody hanging that depended on some override of the previous status quo.

From what I understand this will affect people who want to modifiy the pandoc data and template directories at build time, with like

docker build . [ ... ] --build-arg PANDOC_DATA_HOME=/opt/pandoc

I really doubt that anyone is actually doing that :)

[alerque]

Fair enough. I say good to go on that concern.

Now just pending the syntax issue @tarleb spotted and merge conflict resolution.