Add plugins for package block and package version

[zmughal]

Fixes #9.

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Seems that the only difference between running both in a different order is a
small amount of white-space:

$ perl -I lib -MBabble::Filter=::PackageBlock,::PackageVersion -0777 -pe  babble <(echo 'package Foo::Bar v1.2.3 { 42 }')
{ package Foo::Bar;
our $VERSION = 'v1.2.3';
 42 }
$ perl -I lib -MBabble::Filter=::PackageVersion,::PackageBlock -0777 -pe  babble <(echo 'package Foo::Bar v1.2.3 { 42 }')
{ package Foo::Bar;
our $VERSION = 'v1.2.3';
42 }

I'm debating whether to use a flag to place $VERSION on its own line or not.
And I can also see a flag for using version objects or not.