Option to specify fields by number

[maxharlow]

Some CSV files don't have headers, and it's somewhat awkward to add them just to use csvlink. So something like:

$ csvlink file1.csv file2.csv --field_numbers_1 3,4 --field_numbers_2 1,2

Would also be great to have the option of single-letter arguments to make everything more concise. Also the long argument names should probably use hyphens instead as in other Unix utilities.

[fgregg]

I like both of these ideas.