Add @reexport macro

[simonster]

This would fix #1986. Syntax is:

@reexport using MyModule1, MyModule2

Unlike my macro in #1986, this handles relative imports. It's a little awkward, since the macro expands into using and eval statements, but I couldn't find another way to do it. We might still want a new language keyword instead, but I figured I'd submit this as a PR and see what people think.

[johnmyleswhite]

Having this functionality in any form would be great.

[stevengj]

I'd rather have this as a new keyword...

I also prefer exportall (to mirror importall, and also beginning with export makes it easier to find via tab-completion).

[tknopp]

Also would prefer a new keyword for consistency reasons.

[simonster]

A modified version of this macro is now in METADATA as Reexport.jl until #1986 is resolved.