append(), prepend(), before(), after() support for ZeptoCollection arg

zepto.d.ts defines `append()`, `prepend()`, `before()` and `after()` methods for `string`, `HTMLElement` and `HTMLElement[]`, but Zepto also supports passing in `ZeptoCollection` as an argument. 

I've added two additional method overloads that define the support for passing in `ZeptoCollection`.