Support \operatorname. Resolve #145 (#866)

* Support \operatorname. Resolove #145

* Fix quote mark typo

* Fix test escapes

* Remove poorly written tests

* operatorname screenshots

* Remove nits

* Use documentFragment. Eliminate macro.

test/screenshotter/ss_data.yaml

@@ -172,6 +172,11 @@ OldFont: |
       \text{\rm rm} & \text{rm} & \text{\it it} & \text{\bf bf} & \text{\sf sf} & \text{\tt tt} \\
       i\rm r\it i & \text{r\it i\rm r}
     \end{matrix}
+OperatorName: |
+    \begin{matrix}
+        \operatorname g (z) + 5\operatorname{g}z + \operatorname{Gam-ma}(z) \\
+        \operatorname{Gam ma}(z) + \operatorname{\Gamma}(z) + \operatorname{}x
+    \end{matrix}
 OpLimits: |
     {\sin_2^2 \lim_2^2 \int_2^2 \sum_2^2}
     {\displaystyle \lim_2^2 \int_2^2 \intop_2^2 \sum_2^2}