Base argument

[guybedford]

Per Joel Denning's request to permit emitting an import map that has a separate base to its referenced modules:

~/c/j/p/app1 (main|✔) $ jspm map -o lib/app1.importmap --map-base http://localhost:9001/
err (jspm) Unable to reference mapping @empty at ../../jspm_packages/npm/@jspm/core@1.1.1/nodelibs/@empty.js. The base for the import map must a higher path than its mappings.

I believe the concept is that the base URL of the import map in production will have different relative URLs than the import map during development to the modules in play, so we want a "productionBaseURL" to be inlined as absolute.