Provide an alternative to style injection for CSP purposes

[adcoelho]

I have run into a problem when trying to introduce Content Security Policy (CSP) in my project where, by default, the Diff Match Patch javascript library injects CSS directly into the DOM.

For webpages secured using CSP, this requires to allow style-src 'unsafe-inline' which kind of defeats the purpose of having such policy.

With this PR we will provide an alternative for stricter CSP environments.

The style injection can now be turned off with:

// Disable automatic style injection
diff_match_patch.Style_Injection = false;

and the following CSS file needs to be manually added to the webpage:

<link rel="stylesheet" type="text/css" href="path/to/diff_match_patch.css">

If merged, I can then update the Javascript portion of the wiki with relevant instructions.

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[adcoelho]

@googlebot I signed it!

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CLAs look good, thanks!

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