gitstats: Strip trailing whitespace
Strip blankline before and after usage text

Turns:

    $ ./gitstats

    Usage: gitstats [options] <gitpath..> <outputpath>

into:

    $ ./gitstats
    Usage: gitstats [options] <gitpath..> <outputpath>
Make command arguments optional

This changes the usage such that default values for arguments may be
assumed:

    $ gitstats
    # Build report for current directory into gitstats/ dir

    $ gitstats report
    # Build report for current directory into report/ dir

    $ gitstats foo report
    # Build report for foo/ directory into report/ dir
Change PREFIX to ~/.local