Improve bash completion

[tradej]

Now that #315 has been solved, it is hardly perfect. Here are some suggestions on how to improve it:

• Custom actions should have their own autocompletion lists (e. g. pkg info --installed should only autocomplete names of installed DAPs, etc.). This is simple to achieve - when the appropriate element (assistant or action object) has been found, DA would look into a dictionary with object types and custom completion methods, and upon a succesful find would execute the custom completion method.
• Completion should honor the number of positional arguments, and nargs a flagged argument has.
• Performance improvements
• There should be a comfortable way to temporarily switch bash completion on for ./da.py. Technically, it is simple - only replace da with ./da.py in distribution/da.bash and executing the file in bash. This could be done as a setup.py action, maybe?

More ideas for improvements welcome.