Support for configuration, templates, and projects folder in different folders

Description

It should be possible to have things like configuration files, templates and the projects folder outside the repository. This would make easier to:

• Handle upgrades and migrations.
• Run Onionspray inside containers.
• Do tests.

Currently it's possible to use symlinks for achieving this, but that's suboptimal.

Tasks

• Make Onionspray look up for configurations, templates and the projects folder in different places:
  • Starting from the local repository working copy.
  • Then checking upward, from $XDG_CONFIG_HOME/onionspray to /etc/onionspray.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)