Filtering can be specified using an arbitrary number of -x (or --exclude)
parameters. The filtering works in a similar fashion to an inverted grep,
meaning that matched file names will be excluded from the generated
statistics.
To accommodate this modification, the changing of working directory (when
a repository is specified) has been moved from the system module to the
main gitinspector module. This enables us to change directory in one go
when gitinspector starts, thus removing the need for the previous system
module.
