Allow predicates in unpack to act on string names instead of symbols

[ablaom]

This PR resolves #1043.

After this PR:

julia> table = DataFrame(x=[1,2], y=['a', 'b'], z=[10.0, 20.0], w=["A", "B"])
2×4 DataFrame
 Row │ x      y     z        w      
     │ Int64  Char  Float64  String 
─────┼──────────────────────────────
   1 │     1  a        10.0  A
   2 │     2  b        20.0  B

julia> YW, _ = unpack(table, in(["y", "w"]); string_names=true)
julia> YW
2×2 DataFrame
 Row │ y     w      
     │ Char  String 
─────┼──────────────
   1 │ a     A
   2 │ b     B

[ablaom]

I'm thinking there is a better solution. See the discussion at #1043 .