add groupby and docs

[pdeffebach]

Adding a @groupby convenience macro has been suggested before (though I can't find the exact issues). Currently, you have to write

@chain df begin
    groupby([:a, :b])
    @transform ...
end

The extra [ and the lack of @ is frustrating and gets you out of the "flow" of a chained operation.

This PR allows

@chain df begin
  @groupby :a :b 
  @transfrom ...
end

which is cleaner

This macro also provides a few convenience syntaxes

1. Allows for mixing String and Symbol column selectors
2. Allows for "block" format
3. Automatically concatenates to allow for scalar and vector selection

Unlike DataFrameMacros.jl's @groupby is does not allow for new column creation. This is intentional. I don't want @groupby to create any copies or have any ambiguity about when it creates a copy, which DataFrameMacros.jl's @groupby does (see here)`.

With my implementation, users can mindlessly write @groupby without worrying about performance at all.

[bkamins]

Looks good, but I would recommend to make it more precise how args... are transformed to a single argument to groupby as it was not fully clear to me. What if someone e.g. writes
@groupby(df, Cols(r"a"), Not("b")).

[pdeffebach]

@bkamins Ready for a review.

I opted against adding any escaping rules for @groupby df x. @groupby df x y is now exactly the same as groupby(df, Cols(x, y)). No $ or anything else needed.

1. This corresponds with @by, which is our other maco which allows grouping.
2. This allows for @groupby df [:a, :b] easily. Users don't really need to do [:a, :b] in @groupby when they could just do @groupby df :a :b, but they do need to do that in @by, which doesn't allow grouping in multiple expressions.
3. Why isn't [:a, :b] special-cased in Allow for Between, Not, All, Cols in @select #372, which allows Cols, All() etc? Maybe it should have been, but 1. [:a, :b] might actually signal an actual transformation somewhere and doesn't unambiguously mean a column selection. [:a, :b] and [i for i in ...] actually lower to different expressions and that seems confusing.
4. It's simpler and easier to explain to users.

[pdeffebach]

Thanks!