Add handling of linter messages when no linters for a file type exist (r...

...esets the linter indicator).

Fixes #66.
I like this approach, looks clean.  It maybe better for the manager to trigger the event directly because the reporter is intended to show the marks and stuff on the document.  And maybe call the message "linterUnavailable" or "linterNotFound".  "lintUndertermined" seems like we don't really know what's going on, which isn't accurate.