Improve commands help messages

[jooola]

TL;DR

The following two --help outputs have inconsistent usage description:

Usage:
  hcloud server attach-to-network [FLAGS] SERVER

Usage:
  hcloud network add-subnet NETWORK FLAGS

Order of flags and arguments should not matter, but for consistency, we should always propose the same format in the help output.

Another details, is that sometimes we use [FLAGS] with brackets marking it as optional, and sometimes not. But according to the further output of hcloud server attach-to-network --help the flags here are not optional either, it lists --network as a required flag.

We might also want to stop yelling at our users, and not use UPPERCASE keywords unless really necessary.

Expected behavior

We could use the http://docopt.org/ flags and arguments description language to follow a well known convention.

Related to #700