Artifacts config

This is a work in progress to eventually fix #87 (closed)

At the moment, all it does is let a project configure its pull-url/push-url/push-port, overriding the default user config.

A few caveats/questions:

1. I haven't actually tested this yet 😛

2. Do we want the project config to have priority over the user config (what I did here) or the contrary?

3. The push-port setting is a bit more problematic; indeed what this code does is get the values from the project config, and if they evaluate to False (e.g empty strings which are the default urls) then it takes the value from the user config. But the default push-port is 22, which doesn't evaluate to False. In effect the port will never fallback to the user config.

   I'm thinking we either need to leave the port empty by default, then the code using it is responsible for the port = port or 22, or we could merge the port setting with the push-url one, something like push-url: user@host:port:path, which is a bit annoying: there would be only one : to split on when the default port is used, but two otherwise.

At the moment this doesn't implement the second part of #87 (closed): per-project configs in the user configuration file.