Adding cda tab completion and list command

[t-reents]

This would close #11.
In addition to the list command, I also added tab completion for the cda command to suggest the available projects.

The format of the list command can probably be adjusted. Moreover, one could think of adding additional information about the projects (e.g. a description attribute, etc.).

I was also thinking that it might be useful to add another command to show the configuration of aiida-project. E.g. one could list the project path etc., so something similar to verdi status in aiida-core.

[mbercx]

Thanks @t-reents, and sorry for keeping these nice additions open for so long. We've adapted our approach to supporting various shells, so this PR will also require some updates. If you have time, could you:

1. Keep this PR limited to only adding the list command, which is easier, and open a new PR for the tab-completion?
2. Support tab-completion for all supported shells: bash, zsh an fish.

If you're busy, I understand. I'll get around to it after my sprint on aiida-quantumespresso and qe-tools. :)