Custom failure file (for editor and workflow integration)

[e2]

It would be nice if Guard::RSpec could be configured to leave a "failures file" in a format usable by other tools.

By creating such a file only during failures, Guard could be watching that file and running other tools as a response (e.g. launchy, opening errors in emacs, etc.).

Currently, RSpec has a built-in html and json formatter, but those are AFAIK created every time, and they turn off the default output anyway (so you can't watch the RSpec output and then have a custom output file at the same time).

(Recently, RSpec grew a '--only-failures' option, though I don't know how it behaves in edge cases.)