Fix parsing of .po files for multiline msgstr
Fix parsing of .po files for multiline msgstr

It's valid for a msgstr to be broken over multiple lines, all of which
are enclosed with ".  To handle this, no longer iterate over the lines within
an item but rather apply the matching to the entire item.

mobiata/twine,mobiata/twine,FilamentLabs/twine,FilamentLabs/twine