add docs on js-in, js-debugger

[shaunlebron]

just noticing that there are things in javascript that you can't do with ClojureScript's interop forms, noticing a js-<name> pattern here:

• [x] js-delete - delete object
• [x] js-in - javascript's "in" operator
• [x] js-debugger - breakpoint here in most browser debuggers

source code

[shaunlebron]

• http://cljs-website.github.io/api-docs-report/#cljs.core_js-delete
• http://cljs-website.github.io/api-docs-report/#cljs.core_js-in
• http://cljs-website.github.io/api-docs-report/#cljs.core_js-debugger