Startup script should source plugin profiles

[stuartpb]

It needs PLUSHU_ROOT and PLUSHU_DIR predefined, of course, but PLUSHU_APPS_DIR should be derived from that.

Also, what about sourcing .plushurc? (I'm thinking no)

[stuartpb]

Actually, now that I think about it, Plushu's profile-loading should be an open script for anybody to source if they're going to be an alternate entry point into the Plushu environment. It:

• figures out default values for PLUSHU_ROOT, PLUSHU_USER (this logic, unfortunately, has to be duplicated)
• figures out default values for PLUSHU_DIR, and PLUSHU_PLUGINS_DIR
• loads .plushurc as appropriate, and fails if it's missing
• sources all plugin profiles for their default values, passing them their name and path

[stuartpb]

So, the groundwork for this has been completed - I'm classifying this as a "non-breaking bug", since it's less an "enhancement" and more a "broken assumption that coincidentally doesn't cause problems, yet".