Install extensions using the Command Line Interface

It would be really helpful to have a standardised way to install extensions into the microscope server. This OFEP sets out how that should work. I've folded some of the discussion threads into this description to keep things manageable. Issues to figure out include:

• Add a mechanism for pre- and post- installation scripts (e.g. to install dependencies with apt or make system-level config changes.
• Add a mechanism to declare compatibility (or not) with particular server versions.
• Figure out a way for extension dependencies to coexist with server dependencies
• Figure out a solution for extensions to survive ofm upgrade