better support for across()

[TSchiefer]

The problem was only that we're using enquo() with the option .named = TRUE.
I am already apologizing for the clumsy way of detecting across( with grepl() (which could lead to a problem if someone calls their column weirdly), and I'd be happy to get a pointer here.

I didn't do anything about key tracking, since that will be covered in #663

closes #640

[krlmlr]

Can we let mutate() and summarize() do their thing (i.e. pass ... without really analyzing them) and track keys after the fact? For summarize() we should really only look for grouped columns, for mutate() we give more responsibility to the user.

[TSchiefer]

That makes sense, in line with #663.

[TSchiefer]

I'll do this in a new PR relating to #663.