Warn on command execution using custom styled format

[sgillies]

Resolves #651.

Here's what the short warning looks like:

And here's what we see in planet --help:

$ planet --help
Usage: planet [OPTIONS] COMMAND [ARGS]...

  Planet API Client

  Attention!

  You are using version 1 of the Planet CLI. A pre-release of version 2 is
  available at https://pypi.org/project/planet/. Version 2 will not be
  backward compatible. Changes are planned for the arguments, options, and
  return values of subcommands. For more information about the new version,
  please join the discussion at https://github.com/planetlabs/planet-client-
  python/discussions.

Options:

[sgillies]

Previously, the warning was sent to the terminal using click.echo, but this doesn't provide an easy way to disable the warning. Using Python's warning system, as above, lets us use PYTHONWARNINGS=ignore.

[sgillies]

Now, the warning will point to the module that imports planet.api instead of to planet.api itself.