Added support for replacing args in other expressions than calls

[FNj]

Obvious from added tests:

@test macroexpand( :(@pipe a |> _)) == :(a) #Identity works
@test macroexpand( :(@pipe a |> _[b])) == :(a[b]) #Indexing works

Which means this works:

@pipe 42 |> _ # produces 42
@pipe ["first", "last"] |> _[2] # produces "last"

[FNj]

I also ended up effectively adding support for piping into macros and blocks by removing Expr.head checking in the rewrite(ff::Expr,target) method.

[oxinabox]

delete the Test from REQUIRE, see #10.

[oxinabox]

Hi, thanks for this, seems like a good idea.

can you rebase this PR.
And use _macroexpand rather than macroexpand.

[FNj]

Everything should be fine now. Also I basically hard forked this because I wanted a slightly different behaviour.

[oxinabox]

Oh sorry, I forgot about this PR.

[oxinabox]

LGTM