Bump to phoenix_html v4.0

[PJUllrich]

Hey there,

I like this library and would love to use it, but unfortunately, I'm on phoenix_html: "~> 4.0.0" which removed the Phoenix.HTML.Tag module that this library depends on. Do you have plans on upgrading anytime soon? It would be tricky to support both v3 and v4 of Phoenix.HTML at the same time though 🤔 Let me know if you want help :)