[suggestion] check for updates

[twhitbeck]

This library looks really neat! Thought I'd share an idea. No pressure to implement. If I get the time I'll turn it into a PR.

• Provide a way to check for updates to upstream components. It'd be neat if there was an efficient way to do this, etag checking or last-modified check or something.

This would require tracking which components were installed from where. If there was an updated component, it might be useful to be able to view the diff, but then again you could always just overwrite and then use git to see the changes.

[jacobparis]

My best idea here is to fetch a new component, run it through the transformers, and then apply a unified diff from your code to the new code

This should trigger a merge conflict state in the IDE, so the user can accept new changes while merging any of their own modifications.

As a one-off command, this should just work, but for an automatic check we'd need to store the version of the last check somewhere so it doesn't keep requesting you reconcile your local changes. This might mean the sly.json would contain a "package json dependencies" style array with version numbers. (the registry supports version numbers but they're currently unused and just hardcoded to 1.0.0 for all items and all registries)