improve import matching

[blake-mealey]

Currently the import command matches very few resources, which results in lots of recreated resources after importing. There are a couple of routes we can take with this, and maybe we even want a hybrid of both:

• We could go down a more complex auto-matching route which downloads existing resources and compares them to desired resources.
• We could let the user manually import each matching resource and directly tell Mantle which ones match. This is what Terraform does (see https://www.terraform.io/cli/import).

[blake-mealey]

Adding this to milestone v1.0.0 because import should be considered stable for production use