Handle [package ...] arguments like most Go tools.
Handle [package ...] arguments like most Go tools.

Support import path patterns, including within relative import paths.
When given 0 arguments, use the package in current working directory.
If the import path patterns match no packages, print a warning and
exit with 0 exit code.

Rely on go/loader to resolve relative import paths relative to current
working directory.

Fixes #4.

mdempsky/maligned