Add methods to get a subset of columns

[GlenHertz]

Maybe it is my unfamiliarity with working with NamedTuples but I found it awkward to get a subset of columns when I was porting my DataFrames code to use TypedTables. I came up with the following functions:

https://github.com/GlenHertz/TypedTables.jl/commit/388cadf8d7d92f7ee18d1dc53055cb3b76befec7

I'm not sure if indexing a column by Int makes sense but I do like indexing by Symbols.

Let me know if you think these are useful or if there is an alternate way to do it. I also created ncolumns to get the number of columns.