Automatic release update warnings. (#1336)

* Hide pretty help

* Automatic release update warnings.

* `httpie cli check-updates`

* adapt to the new loglevel construct

* Don't make the pie-colors the bold

* Apply review feedback.

Co-authored-by: Jakub Roztocil <jakub@roztocil.co>

httpie/manager/cli.py

@@ -24,6 +24,9 @@ COMMANDS = {
                 'default': 'json'
             }
         ],
+        'check-updates': [
+            'Check for updates'
+        ],
         'sessions': {
             'help': 'Manage HTTPie sessions',
             'upgrade': [