Allow log parsing without any log_line_prefix and extend it to be defined by the...

Allow log parsing without any log_line_prefix and extend it to be defined by the user. Custom log_line prefix can be parsed using user defined regex with command line option
--regex-db and --regex-user. For exemple the default regex of pgbadger to parse user and db name from log_line_prefix can be written like this: pgbadger -l mylogfile.log  --regex-user="user=([^,]*)," --regex-db="db=([^,]*)"