Support for top-level \newline and \\ in inline math (#1298)

* Support for top-level \newline and \\ in inline math

This was a little tricky because `\\` was defined as an endOfExpression.
Instead made `\\` a termination specific to an array environment.
Outside an array environment, buildHTML handles the `cr` object,
resulting in a `.newline` class.  Currently this turns into a
`display: block` (with appropriate vertical spacing) only in inline math,
matching LaTeX.

* Simplify code

* Fix Jest errors

* NewLine screenshot test

* Bug fix: \\ only works at top level of inline

* Add \newline and \cr to test

* Switch test to pmatrix

* Add vertical space test

* Add \\ vs. \newline tests

* Fix flow errors

* Add \cr test

* Add documentation for \\ at top level

* Comment out newRow

* Fix commenting out

README.md

@@ -132,6 +132,13 @@ will appear larger than 1cm in browser units.
 - MathJax supports Unicode text characters in math mode, unlike LaTeX.
   To support this behavior in KaTeX, set the `unicodeTextInMathMode` option
   to `true`.
+- KaTeX breaks lines with `\\` and `\newline` in inline math, but ignores them
+  in display math (matching LaTeX's behavior, but not MathJax's behavior).
+  To allow `\\` and `\newline` to break lines in display mode,
+  add the following CSS rule:
+  ```css
+  .katex-display > .katex > .katex-html > .newline { display: block !important; }
+  ```
 
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