Adding ability to configure conda-store via Traitlets

[costrouc]

Considered several options for making conda-store configurable. Ultimately the python configuration that is possible via traitlets allows for heavily customization that would be hard without it.

Packages worth looking at:

• dask-gateway
• jupyterhub
• jupyter-ssh

These can show how to configure traitlets in a way that is convenient to end users.

[costrouc]

This will be an issue that will encompass much of the code. Currently cli options are used to configure conda-store. However, over time is has become apparent that there will be too many cli options. Thus is would be nice to use traitlets to configure all of these options.