Introduce case-insensitivity for reference link labels (JohnSundell#16)

Merge PR from @ezfe
* Introduce case-insensitivity for reference link labels
* Moved `.lowercased()`

HTML: Treat comments as self-closing tags (JohnSundell#10)

This patch makes Ink parse HTML comments correctly, by treating them as
self-closing tags, rather than as opening ones (which would cause all
subsequent text to be parsed as plain HTML).

Merge pull request #1 from JohnSundell/linux-ci

Linux: Include all test cases

Add LICENSE file