Update codebase to current Crystal

[devnote-dev]

A lot of the codebase seems to be using code that was likely before the Crystal 1.0 release, and while parts of the library have been updated in recent-ish updates, a lot of it still uses old or outdated code/practices. My proposals are:

• https://github.com/icyleaf/markd/pull/58
• Replace the HTML extensions/mappings with the standard library's HTML module
• https://github.com/icyleaf/markd/issues/54
• https://github.com/icyleaf/markd/issues/55
• https://github.com/icyleaf/markd/issues/56
• Revisit specs coverage

There are few other things that I would like to propose but I will move them into separate issues. Let me know what you think.