#78 prefilter option

[Jarek Czekalski]

As suggested in [url=https://sourceforge.net/projects/ctags/forums/forum/20308/topic/4638807]Add --run-filter switch[/url] forum subject, a filtering option could be useful. Here is a patch that introduces such an option.

Here's output of the usage note:

--prefilter=program
Specify a pathname of a program that will act as a filter.
The output of <program> <orig_file> will be processed instead of
the original file.

[Jarek Czekalski]