Extend `autocomplete` attribute values

[aronerben]

Hello, thanks for this fantastic project!

Currently, the autocomplete attribute only supports on and off. The spec lists many more options. It would be nice if Tyxml supported those too! I don't know if this is already on the roadmap, or similar, so I decided to open this issue. :)

Encoding all possibilities mentioned in the spec with types seems a bit tedious (as there seem to be named groups, special cases for hidden input fields, etc.). Maybe simply turning it into a string attribute is "good enough" (I don't know what Tyxml's philosophy is here).

Currently we've solved it by unsafely adding the attribute like so:

let open Tyxml_xml in
let open Tyxml_html in
street_input
|> toelt
|> amap1 (fun _ attrs ->
      add_string_attrib "autocomplete" "street_address" attrs)
|> tot)

where street_input is some input field.

[Drup]

That specification is indeed pretty awkward, but I would prefer to avoid "just using a string". In this case, it seems we could start with a fairly simple model such as `On | `Off | `Tokens of string list and see where that leads us. In any case the ppx extension can check things in more details.

If you are willing, don't hesitate to submit a (potentially WIP) implementation ! There are instructions on how to add/change attributes in https://github.com/ocsigen/tyxml/blob/master/docs/Contributing.md . :)

[aronerben]

@Drup Thanks for the quick response! I've drafted a PR: https://github.com/ocsigen/tyxml/pull/303

[Drup]

Merged !