update the privacy policy to mention LaTeX rendering service; make La…

…TeX support off by default

This is prompted by PR#877, where a user pointed out that the caveat should be present.

I am uncomfortable enough with this that I'm making the feature off by default. If #874 (local LaTeX rendering) ever gets implemented, this should be again made on by default.

PRIVACY.md

@@ -6,3 +6,7 @@ This means that:
 - It collects no user or usage data.
 - It does not read or modify any part of any site besides the contents of the focused edit control, and only when the user tells it to do so.
 - It makes no external requests at all. All rendering is done locally.
+
+## Optional third-party: LaTeX image rendering service
+
+If you choose to use _Markdown Here_'s LaTeX math rendering feature, the contents of your math equations will be sent to a third party to create the image containing your math. (That image is created at the time that the browser displays it, which is usually when the image element is inserted into the page). By default that third-party service is [CodeCodgs](https://www.codecogs.com/pages/agreements/privacy_policy.php) although [other services](https://github.com/adam-p/markdown-here/wiki/Tips-and-Tricks#tex) may be used.

src/common/options-store.js

@@ -17,7 +17,7 @@ if (typeof(Utils) === 'undefined' && typeof(Components) !== 'undefined') {
 
 // Common defaults
 var DEFAULTS = {
-  'math-enabled': true,
+  'math-enabled': false,
   'math-value': '<img src="https://latex.codecogs.com/png.image?\\dpi{120}\\inline&space;{urlmathcode}" alt="{mathcode}">',
   'hotkey': { shiftKey: false, ctrlKey: true, altKey: true, key: 'M' },
   'forgot-to-render-check-enabled': false,