Implement strict mode (replacing unicodeTextInMathMode) (#1278)

* Implement strict mode (replacing unicodeTextInMathMode)

Add new "strict" setting (default value false) that can take a boolean
(whether to throw an error or silently ignore), string ("ignore",
"warn", or "error"), or a function possibly returning such a value.
This enables a variety of ways of handling or ignoring transgressions
from "true" LaTeX behavior, making KaTeX easier to use while still
providing the ability for strict LaTeX adherance.

Resolve #1226, implementing that spec, for two existing
transgressions from regular LaTeX:

* src/functions/kern.js had some errors and warnings about use of
  (units in) math vs. text mode commands.
* The former setting unicodeTextInMathMode (not in any released version)
  needed to be set to true to enable Unicode text symbols in math mode.

Now these are controlled by the strict setting.  By default, KaTeX is now
very permissive, but if desired, the user can request warnings or errors.

* Rewrite strict description

* Add tests for strict functions

* Stricter type for strict

* Switch default strict setting to "warn"

* Fix new flow error

* Fix another flow bug

static/main.js

@@ -28,6 +28,17 @@ function init() {
         options.displayMode = false;
     }
 
+    // Use `strict=warn` for warning strict mode or `strict=error`
+    // (or `=1`/`=t`/`=true`/`=y`/`=yes`)
+    // to turn off displayMode (which is on by default).
+    if (query.strict) {
+        if (query.strict.match(/^(1|t|y|e)/)) {
+            options.strict = "error";
+        } if (query.strict && query.strict.match(/^(w)/)) {
+            options.strict = "warn";
+        }
+    }
+
     // The `before` or `pre` search parameter puts normal text before the math.
     // The `after` or `post` search parameter puts normal text after the math.
     // Example use: testing baseline alignment.