Interface for choosing startup directory

[davidlatwe]

I am thinking instead of providing an interface for modifying environment vars like #113 proposed, what about an interface for formatting startup directory path ? Which could be more generally useful.

The main benefit would be, implementing rezconfig hierarchical override in pipeline can be much easier (so that you can e.g. have different package filter when entering different workspace of shot/asset).

The path template and the arguments (maybe) can be provided by the profile package, but not sure how the interface would look like. Any thoughts ?