Introduce an aligned environment

This is almost like the align* environment, but it starts out in math mode,
so we don't have to worry about the fact that we have no real surrounding
text mode in KaTeX.  This is the first step towards align* and align.

test/screenshotter/ss_data.yaml

@@ -12,6 +12,11 @@
 # on the next line. See http://www.yaml.org/ for syntax details.
 
 Accents: \vec{A}\vec{x}\vec x^2\vec{x}_2^2\vec{A}^2\vec{xA}^2
+Aligned: |
+    \begin{aligned}
+        a &= 1 & b &= 2 \\
+        3a &= 3 & 17b &= 34
+    \end{aligned}
 Arrays: |
     \left(\begin{array}{|rl|c||}
         1&2&3\\