New Instance Model

[michaelquigley]

In v0.3, instances are located in ~/.fablab/instances and the active instance is selected with the fablab use command.

In v0.4 we should consider creating an instance directory relative to the current directory (or a provided path) when invoking the fablab create command, and then selecting the active instance is done either from the process' working directory, or possibly overriden with a comand-line flag or an environment variable.

[plorenz]

Completed. Create now takes an optional working directory. The fablab config.yml tracks working directories for each instance.