Add sorting option for export

[ckindermann]

This implements #20

[ckindermann]

We already use -s as a flag for --streaming. I used -l as a flag for --sort (because it's a lexicographic sort). Also, the options --streaming and --sort cannot be combined. This is not an issue per se, but if we use --sort by default, then users will expect this sorted output. So, we should probably warn users about outputs not being sorted when using --streaming if --sort is configured as a default. Alternatively, we don't make --sort a default and throw an error when users set both --steaming and --sort (this is the current behavior).

[jamesaoverton]

That fix worked for me. Thanks!