Support \cfrac (#1392)

* Support \cfrac

This was an easier PR to write than #135 since KaTeX now has well established methods for pt-to-em conversions and nulldelimiter.

As in #135, this does not support the LaTeX optional argument for numerator justification.  It supports a `\cfrac #1 #2 ` syntax.

* Added screenshotter files

* fix ref to context in genfrac handler

test/screenshotter/ss_data.yaml

@@ -106,7 +106,7 @@ ExtensibleArrows: |
         \xrightleftharpoons[ab]{ABC} + \xhookrightarrow[ab]{ABC} \\
         \xtwoheadrightarrow{ABC} + \frac{\xrightarrow[ab]{ABC}}{\xrightarrow[ab]{ABC}} + \left\lvert\xrightarrow[ab]{ABC}\right\rvert
     \end{array}
-FractionTest: \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12\;{1 \atop 2}
+FractionTest: \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12\;{1 \atop 2}\; \cfrac{1}{1+\cfrac{1}{x}}
 Functions: \sin\cos\tan\ln\log
 Gathered: |
     \begin{gathered}