Allow for keyword arguments

[pdeffebach]

This just adds keyword arguments for calls of the form @subset(df, :a; view = true), rather than the multi-line version

@subset df begin 
    :a 
end

I have added this for @subset but not others yet.

[pdeffebach]

@jkrumbiegel can I get a quick check on the implementation? It borrows from DataFrameMacros.jl.

With your approval I will add the same structure to everything else.

[jkrumbiegel]

Yeah looks ok from a quick glance. I still haven't decided how keyword arguments could work for non-parentheses style.

[pdeffebach]

I guess the current code isn't very DRY, but each 4-line *_helper function is unique and it seems overly abstract to make an outer function right now.

Will plug away with tests.

[pdeffebach]

I think I have the pipeline figured out for how to process arguments. I've written it for @rtransform but need to port it to the rest of the macros.

[bkamins]

Nice!

[pdeffebach]

Okay! I think this is all done @bkamins @nalimilan @jkrumbiegel

Please take a look!

[pdeffebach]

Thanks!

[pdeffebach]

Okay I'm ready to merge this. There will always be more docs to add but I think this is now pretty comprehensive.