Add additional types to features

[cinxmo]

Once #40 merges, all 3 "types" will be considered file attachments:

• local imports
• local fetches
• actual file attachments (created when we call FileAttachments(...) within a cell)

This causes an issue because import files would be available within the inputs of a certain cell, but it is not needed. We only promoted local imports to file attachments because we want them copied over during the build step.

The proposed solution is to define other feature types (LocalFetch, LocalImport) and modify the code to explicitly check for certain types

[mbostock]

This causes an issue because import files would be available within the inputs of a certain cell, but it is not needed.

Why is this an issue? What is the harm in allowing the files to be fetched, if desired, by implicitly promoting them to file attachments? The code must be included with the built site, so there’s no reason to prevent them from being loaded if desired.

[mbostock]

I closed this fixed by #120, which made it so imports are no longer promoted to file attachments, and added type safety. We could something similar for local fetches, but I’m not sure anything is needed. PR is welcome though!