Support \colorbox and \fcolorbox (#886)

* Support \colorbox and \\fcolorbox

These are functions from the `color` package. They accept text, not math. They also have padding similar to `\fbox`.

Because of the padding, the code in `buildHTML` is intermixed with the `\fbox` code. I have not (yet) created a new file in the functions folder. This way, the reviewer gets a cleaner diff.

* Fix lint error

* colorbox screenshots

* Pick up review comments

static/katex.less

@@ -593,6 +593,10 @@
         box-sizing: border-box;
         border: 0.04em solid black;   // \fboxrule = 0.4pt
     }
+    .fcolorbox {
+        box-sizing: border-box;
+        border: 0.04em solid;   // \fboxrule = 0.4pt
+    }
     .cancel-pad {
         padding: 0 0.2em 0 0.2em;    // ref: cancel package  \advance\dimen@ 2\p@ %  "+2"
     }