fftools/cmdutils: add support for level flag in loglevel option parser

Allows to manage the AV_LOG_PRINT_LEVEL flag as a prefix to the loglevel
option value, similar to the existing AV_LOG_SKIP_REPEATE flag. Adds
support for setting flags relative to the existing value by using a +/-
prefix.

Previous version reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Tobias Rapp <t.rapp@noa-archive.com>