Allow the user to specify additional directories to look for modules to import

[dlespiau]

We've been careful crafting the importing code to not important beyond the script directory. There are some cases where the user wants to specify additional import directories though: if the script is in a sub-directory but need to access library code from a top-level directory.

We could add -I option to tell the various importers to consider extra directories when looking for modules.

• -I added to the common VM object (common, between run and generate)
• Add support for extra include directories to the FileImporter (https://github.com/jkcfg/jk/blob/master/pkg/resolve/file_importer.go). It'd be nice to have a unit-test for that as well in file_importer_test.go.
• Add support to extra include directories to the NpmImporter (https://github.com/jkcfg/jk/blob/master/pkg/resolve/npm.go). It'd be nice to have a unit-test for that as well in npm_test.go.
• Integration tests created