filter-known-issues: add warning support, clean up, add more help

Introduce support to tag some issues as warnings, that can be printed
but not considered errors.

Added the ability to write the errors and warnings to split files, for
later reporting.

Added a more consistent help.

Change-Id: Ia75430c3337385afca59595437168ab643c92911
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>