exclude list for the keybase filesystem

I added a document to an area in the keybase FS, and realised I needed to edit it. As I was editing it, keybase kept sync'ing the editor's temp file up.

It'd be great to have an exclude / ignore file list, where we could add ".*.swp" for example to avoid having the vim swap files sync'ed.

No files, tests, or entry points are named. Start by locating the keybase filesystem implementation and its synchronization path, then determine how an exclude list would represent patterns such as ".*.swp" and how ignored editor temporary files should be verified.

Written by the indexing model from the issue text.