make the extract task accept paths

I.e. make it possible to do:

	Console\cake Translations.extract Plugin/foo

And for it to then extract from only this path (also accounting for the
webroot folder)
don't forget to pass the category
for plural functions - always define {number}

make it consistent. Will otherwise likely be a pending bug for any
language with more complex plural rules
remove incorrect warning

The plugin handles both
correct throw statements
dk is not a language
updated docs