Create slick.headerbuttons.d.ts

Headerbuttons is a SlickGrid plugin so I put it in the slickgrid directory:
https://github.com/mleibman/SlickGrid/blob/master/plugins/slick.headerbuttons.js?source=cc
Changing the context param to be OPTIONAL

The compile function returns a template function. The template function takes two parameters, `context` and `options`. These are both optional parameters. This commit fixes `context` to be optional.
Update slick.headerbuttons.d.ts
Merge pull request #1603 from derekcicerone/patch-35

Create slick.headerbuttons.d.ts
Merge pull request #1605 from styfle/patch-2

Changing the context param to be OPTIONAL