FEATURE: Add filetype or other hook for user mods

[fourjay]

USE CASE: I'd like to add some custom local behavior to the startup-log.txt window (examples: a "q->quit mapping", a "CR -> fold-toggle").

With the buffer strategy of open and then name, there's no event I can see to add customization.

One approach is to set some sort of filetype, say "startuptime-log" (should be specific enough to not interfere with any other filetype). This allows autocmds to trigger off of filetype.