append() / before() support for ZeptoCollection arg

zepto.d.ts defines `append()` and `before()` methods for string, HTMLElement and HTMLElement[], but Zepto also supports ZeptoCollection. Added method overloads supporting ZeptoCollection.
after() allows ZeptoCollection arg
prepend() allows ZeptoCollection arg
Merge pull request #1934 from milkisevil/master

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