Allow mime const initialization with attributes

[yoshuawuyts]

From talking with @dignifiedquire offline about #51, the biggest constraining feature we have currently is not being able to initialize mime types with attributes as constants. E.g.

const TEXT: Mime = "text/plain; encoding=utf-8"

We should figure out how to enable it, probably looking to hyperium/mime as inspiration as their impl does allow for this.

[yoshuawuyts]

Oh, so they don't allow this either. Instead internal fields are populated: https://docs.rs/mime/0.3.16/mime/constant.APPLICATION_JAVASCRIPT_UTF_8.html. We could probably do something similar I guess.