v0.3.0

Opt-in Gutenberg block rendering, for SureCart Cart/Checkout.

SureCart's Cart and Checkout blocks have no shortcode fallback - a new
enable_blocks setting (off by default, no editor UI yet) makes Rawmark
also run do_blocks() on a page's composed HTML before do_shortcode(),
so real block-comment markup (e.g. surecart/checkout-form) renders and
functions instead of sitting inert.

Implemented and unit-tested (204/204 PHP tests green), but not yet
verified live in a real browser against a real SureCart checkout/cart
flow - no browser-automation tool is available in the dev environment
this was built in. If you install this and click through a real
Cart/Checkout with enable_blocks on, feedback on whether it actually
works is welcome.