Add JS integration

[sminnee]

Add support for Raygun's JavaScript tracking, for either front-end code, or the admin.

My (biased) perspective is that we should do this as a https://github.com/sminnee/silverstripe-tagmanager plugin, so that we don't get a bunch of modules each providing their own middleware for modifying the outputted HTML. The status quo, for e.g. google analytics insertion is a bit of a mess in this regard. Keen to get thoughts on that, though.

• A simple interface for defining a snippet, and a middleware for updating the resulting HTML
• An admin UI for selecting & configuring which snippets should be active. This is less relevant for raygun than, say, google analytics or crasyegg, but putting CMS admins in control of potential feature bloat isn't necessarily a bad thing.

On my to-do list:

• Support for adding the same kinds of snippets to the admin if you want
• Potentially the choice between yml snippets and UI-configured ones, if you don't want to put front-end Raygun insertion in the CMS administrator's hands.

[GuySartorelli]

Was the update to sminnee/silverstripe-tagmanager sufficient to cover this use case? If so, it might be reasonable to just update the docs for this module to point to it.