Prepare for 0.9.0 release

[pdeffebach]

This PR does 3 things

1. exports @passmissing for users to use the documentation, ? @passmissing will work even if @passmissing ... won't outside of DataFramesMeta.jl macros
2. Only allows for DataFrames > 1.0, to fix subset not defined #274
3. Bump version to 0.9.0

We need to bump the version because we add new features and are pre-1.0.

[pdeffebach]

@bkamins I looked over some of the references to keyword argments in the documentation. Honestly I don't think it's that bad. @transform df :y = f(:x) is a keyword argument, for the most part. In the absence of having a better idea for a name, I think we should make a release.

[bkamins]

OK. For me x = something is a keyword form, and :x = something is more of an assignment, but I am OK with leaving the keyword argument style. Maybe we could in the first place where we mention it just give an example that by :x = :y we mean that this is a keyword argument style?

[pdeffebach]

Okay after #286 let's merge this and make a release!

[bkamins]

#286 is already merged :)