Question concerning 〜やがる / Imperative in "Slang"


I've come across these expressions in all sorts of anime, manga and light
novels and now I am wondering behind the "mechanics" of it...

> 「勝手に言 **ってろ** 」「ほざい **てろ** 」「食らい **やがれ** 」

In these cases the imperative is used; these expressions are usually similar
to the English 'Shut up', 'Take that', etc.

However in cases like:

> 「 **ふざけやがって** 」

I am not sure what is implied by that...

I get the meaning of what is said however I am unsure about its grammatical
form. Can it be understood as 「っている」 just with 「いる」 being left out?

What exactly does this 「〜て」 mean if nothing follows here?

