4.7 compatibility

Fixes an incompatibility with Symfony 4 components used in Silverstripe 4.7

Enhancement - sitemaps

Adds checks in the middleware to prevent XML sitemaps being minified and potentially having important data stripped out.

Alpha release

Alpha package release. Please report issues at https://github.com/DorsetDigital/silverstripe-htmlminifer/issues

Stable release

Initial stable release