tab completion instructions for bash and zsh (#11)

* instructions how to add completion.

* argument completion won't come until click 7.0

* version bump.

* drop 3.4 support.

* remove python 3.7

* new gif.

* new gif and use limit not count.

.travis.yml

@@ -4,7 +4,7 @@ python:
   - "2.7"
   - "3.5"
   - "3.6"
-  # - "3.7-dev"
+  # - "3.7"
 before_install:
 - "pip install -U pip"
 - "python setup.py install"