Implement basic bash tab completion

If [argcomplete](https://argcomplete.readthedocs.io/en/latest/) is installed,
bash tab completion will be enabled for pmbootstrap's arguments.

If argcomplete is not installed, pmbootstrap continues to work as before.

pmb/__init__.py

+# PYTHON_ARGCOMPLETE_OK
 """
 Copyright 2018 Oliver Smith
 

pmb/parse/arguments.py

@@ -459,6 +459,12 @@ def arguments():
                                  help="force even if the file seems to be"
                                       " invalid")
 
+    try:
+        import argcomplete
+        argcomplete.autocomplete(parser)
+    except ImportError:
+        pass
+
     # Use defaults from the user's config file
     args = parser.parse_args()
     pmb.config.merge_with_args(args)

pmbootstrap.py

 #!/usr/bin/env python3
+# PYTHON_ARGCOMPLETE_OK
 
 """
 Copyright 2018 Oliver Smith

setup.py

@@ -58,6 +58,9 @@ setup(
     packages=find_packages(exclude=['aports', 'keys', 'test']),
     tests_require=['pytest'],
     cmdclass={'test': PyTest},
+    extras_require={
+        'argcomplete': ["argcomplete"],
+    },
     entry_points={
         'console_scripts': [
             'pmbootstrap=pmb:main',