Allow user to enter multiple satellites in command line call

[cgmorton]

I think it would be nice if the user could list multiple satellite options in the command line call. I've done this before using the nargs='+' or nargs='*' options in argparse (https://docs.python.org/3/library/argparse.html#nargs).

So the command might look something like: landsat --satellite LE7 LC8 --start ...