Indexing a ComponentArray with an KeepIndex(array of symbols)

[bgctw]

Similar to #71.
However, if the result is still a ComponentArray, then the behaviour should be well defined and work with matrices and other types too.

This would also allow #71 to work with
getdata(cv[KeepIndex([:a,:b])])

[bgctw]

Thanks, @jonniedie.
How about the edge cases?

   @test ca[(:a,)] == ca[KeepIndex(:a)] # using setup near runtest.jl line 173
   @test ca[[:a]] == ca[KeepIndex(:a)]
   ca[()] #? what is an empty ComponentVector