Add cli support (#1855)

Add cli support (#1855)

* Add cli support

* Add setup.py

* Import main to 'httpx.main'

* Add 'cli' to requirements

* Add tests for command-line client

* Drop most CLI tests

* Add test_json

* Add test_redirects

* Coverage exclusion over _main.py in order to test more clearly

* Black formatting

* Add test_follow_redirects

* Add test_post, test_verbose, test_auth

* Add test_errors

* Remove test_errors

* Add test_download

* Change test_errors - perhaps the empty host header was causing the socket error?

* Update test_errors to not break socket

* Update docs

* Update version to 1.0.0.beta0

* Tweak CHANGELOG

* Fix up images in README

* Tweak images in README

* Update README