Add the validate sub-command

[iawia002]

$ ksbuilder validate test-1.0.0.tgz
validating extension test-1.0.0.tgz

no issues found

$ ksbuilder validate tower
validating extension tower

error executing command: invalid language code zh1, see https://www.loc.gov/standards/iso639-2/php/code_list.php for more details
exit status 1

close #71

[iawia002]

/cc @stoneshi-yunify Take a look and see if this is what you're looking for. Actually, validation is performed before every execution of publish/package. Anyway, I added this sub-command separately.

[stoneshi-yunify]

thanks very much

[iawia002]

I'll release a new version later with another feature.