Symfony 4 compatibility

This patch adds compatibility with the 4.7 Silverstripe release, by ensuring Symfony 4 syntax in the yml config

Version 2 release

Improves flexibility of the rewriter to accommodate changes in the resource directory.

Bugfixes

Improve handing of admin URL detection

Compatibility release

Change middleware naming to avoid collisions

Add theme support

Add theme rewriting support

Fix issue preventing resources from being rewritten when in a subdirectory
Move incorrectly nested condition for prefetch injection

Add prefetch support

Enhancement: Add dns-prefetch support
Bugfix: Check for response before adding headers to ensure build processes aren't interrupted

Subdirectory support

Add subdirectory support
Improve detection of admin routes

Initial release

Update readme

Add licence file