Allow autocomplete within placeholders

[tomak-git]

When using placeholders Light Table's autocomplete function is suspended.

Steps to reproduce:

1. Create a snippet with a placeholder in it.
2. Type the triggering letters, hit Snippets: Expand by editor token.
3. When typing within the placeholder, autocomplete does not work.

It would be great to have access to autocomplete when typing within placeholders.

[rundis]

Without spending to much thought into it I'd say this would require a pretty substantial rewrite. Current implementation just overlays a simple dom widget with a form over the editor, to support autocomplete I'd probably somehow have to implement the overlay as an codemirror document. Not sure how I'd go about doing that and whether it is really worth the effort.

[rundis]

Closing due to low cost benefit ratio and not enough spare time. Feel free to submit a pull request though.