Add --exlcude option

[andras-kth]

I happen to have an index.html in my PACKAGE_DIR and get the following

user@host$ dir2pi .
Traceback (most recent call last):
  File "/opt/homebrew/bin/dir2pi", line 8, in <module>
    sys.exit(dir2pi())
  File "/opt/homebrew/lib/python3.8/site-packages/libpip2pi/commands.py", line 312, in dir2pi
    return _dir2pi(option, argv)
  File "/opt/homebrew/lib/python3.8/site-packages/libpip2pi/commands.py", line 356, in _dir2pi
    pkg_name, pkg_rest = file_to_package(pkg_basename, pkgdir)
  File "/opt/homebrew/lib/python3.8/site-packages/libpip2pi/commands.py", line 159, in file_to_package
    raise InvalidFilePackageName(file, basedir)
libpip2pi.commands.InvalidFilePackageName: unexpected file name: 'index.html' (not in 'pkg-name-version.xxx' format; found in directory: '.')
user@host$

With this PR applied, I can exclude the non-package content:

user@host$ dir2pi -X *.html .
user@host$

This may also resolve #68 and #69 in a more disciplined manner.