copy-tex contrib module (#813)

* copy-tex contrib module

* Factor out replacement code

* Fix lint

* Support for more browsers, in particular Firefox

* Use for loop instead of Array.forEach
* Use replaceChild if replaceWith is unavailable
* Browserify to remove let etc.

* Fix HTML handling, in particular for Edge

* Convert DocumentFragment to HTML directly (via children' outerHTML)
* Set HTML content *before* text content; Edge takes the last only

* Handle .katex-html and .katex-mathml separately

* Implement option 2: CSS user-select: all

Also fix auto-render.js build location

* Revise documentation according to @kevinbarabash's comments

* Split copy-tex.js into it + katex2tex.js

This supports re-use of code in a custom copy handler.

* Document custom copy handler

* Add missing file

contrib/copy-tex/copy-tex.css

@@ -0,0 +1,12 @@
+/* Force selection of entire .katex/.katex-display blocks, so that we can
+ * copy/paste the entire source code.  If you omit this CSS, partial
+ * selections of a formula will work, but will copy the ugly HTML
+ * representation instead of the LaTeX source code.  (Full selections will
+ * still produce the LaTeX source code.)
+ */
+.katex, .katex-display {
+    user-select: all;
+    -moz-user-select: all;
+    -webkit-user-select: all;
+    -ms-user-select: all;
+}