[stub] support alternate HTML theme through environment variable

[michaelmhoffman]

Change to build.sh demonstrating potential solution to #112.

[agitter]

I like this design. We should consider doing the same for CSL_PATH, BIBLIOGRAPHY_PATH, and INPUT_PATH.

[dhimmel]

Noting that if we adopt pandoc --defaults as proposed in #290, this method of setting pandoc options through environment variables will no longer work.

[michaelmhoffman]

PANDOC_DEFAULTS_PATH="${PANDOC_DEFAULTS_PATH:-build/pandoc-defaults.yaml}"

[dhimmel]

We merged #290, which includes something similar to @michaelmhoffman's suggestion:

rootstock/build/build.sh

Lines 22 to 24 in 06fb01d

	# Pandoc's configuration is specified via files of option defaults
	# located in the PANDOC_DEFAULTS_DIR directory.
	PANDOC_DEFAULTS_DIR="${PANDOC_DEFAULTS_DIR:-build/pandoc-defaults}"

Now that we're using --defaults files, this PR is less relevant, so am closing it.

To customize pandoc's behavior, you can either edit the relevant line a default YAML file, or create a new pandoc-defaults directory and change PANDOC_DEFAULTS_DIR as discussed at #290 (comment).