flit with no arguments throws instead of printing useful info

[mikebentley15]

Bug Report

Description

When you run flit with no arguments, an exception is thrown

Traceback (most recent call last):
  File "/Users/michael.bentley/bin/flit", line 250, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/Users/michael.bentley/bin/flit", line 187, in main
    return _main_impl(arguments, module_dir, prog=prog)
  File "/Users/michael.bentley/bin/flit", line 246, in _main_impl
    subcommand = subcommand_map[args.subcommand]
KeyError: None

This isn't super helpful

Suggested Fix

Print the same documentation as flit --help.

Alternative approaches:

Print an error and suggest calling flit --help or flit help.