Take snapshots of formulas rendered by LaTeX and compute visual diffs

The same test cases we use for our screenshots from Firefox are now also
being rendered by pdflatex, so the resulting images can be used as reference
for how things are supposed to look (if we concentrate on compatibility with
LaTeX).  To make comparisons even easier, the differences between LaTeX and
Firefox snapshots are rendered in a visual way, using different colors.

Discussed in pull request #268.

dockers/texcmp/texcmp.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -x
+imgname=katex/texcmp
+tag=1.1
+imgid=$(docker images | awk "/${imgname//\//\\/} *${tag//./\\.}/{print \$3}")
+cd "$(dirname "$0")" || exit $?
+npm install || exit $?
+if [[ -z ${imgid} ]]; then
+    docker build -t "${imgname}:${tag}" . || exit $?
+fi
+base=$(cd ../..; pwd)
+docker run --rm \
+       -v "${base}":/KaTeX \
+       -w /KaTeX/dockers/texcmp \
+       "${imgname}:${tag}" \
+       nodejs texcmp.js "$@"