Change CLI to take packages as parameters, like go list.

Add --stdin flag to read package list from stdin, as before.
Minor refactoring to improve code, reduce dot imports.
Update sample output in README for new CLI.
Rename "--all" flag to "-v".

Be more consistent with go tool, and less confusing when specifying
"all" as the import path pattern.
Print "can't load package" message if invalid Go package is supplied.
Merge pull request #22 from shurcooL/take-packages-as-parameters-like-go-list

Change CLI to take packages as parameters, like go list.