Support custom elements

[YerkoPalma]

This would be similar to what MDX does but for custom elements, and it's pretty simple.
The idea would be to modify only the Lexer class, and reuse the Parser and Rendere for html. I've already made it for a personal component, but I had to copy and paste the whole lex function.
Api would be something like

const md = require('md')

const html = md(`## hello world

<custom-button>Fancy button!</custom-button>

- [ ] todo
- [x] done
`, { customElements: ['custom-button'] })

If you agree I could send a PR