"grepformat" with -l: match filename-only

[blueyed]

When using -l to list only files (e.g. with ripgrep) it would be helpful if the grepformat (used for errorformat) would contain %f to handle the filename-only result.

I can imagine having %f just by default (appended) always might make sense, but could also be added/used only when -l is used. There are other options though that change the output etc, so something more flexible might make sense.

I'm not sure about adding unnecessary complexity here - so this is meant as a start for discussion/ideas.

[mhinz]

Let's go the simple route unless there's a real problem (something that can't be solved by configuration alone), thus just appending %f to the defaults.

As the README states, I won't add any complex features anymore. Only the usual maintaining work like bug fixes and small enhancements.