Filtering of logged data

Required to use Raygun in applications dealing with sensitive customer data that shouldn't be logged off site. Developed as part of client work at SilverStripe Ltd (/cc @sminnee @halkyon @madmatt). Similar feature to the Ruby client:

https://github.com/MindscapeHQ/raygun4ruby/commit/000f97835285c5613dcb8ade1ad7792aaee8f62c https://github.com/MindscapeHQ/raygun4ruby/commit/36cbf44c99abdf67a1a28fc73bdb0e27763553f7 https://github.com/MindscapeHQ/raygun4ruby/commit/e34df7faeb1b8f8a196136e32391922d28b985d3 https://github.com/MindscapeHQ/raygun4ruby/commit/3bb2f9868380f0a4e2fe4713c9c19fbfb9785d88