allow the msgid to be passed as attribute instead of as html

without modification switching to key_ids the my-i18n directives would
look something like this:

> <h5 my-i18n>im_contacts_header</h5>
> <h5><my-i18n>im_contacts_header</my-i18></h5>

but now can also be written as

> <h5 my-i18n="im_contacts_header"></h5>
> <h5 my-i18n msgid="im_contacts_header"></h5>

> <h5><my-i18n msgid="im_contacts_header"></my-i18></h5>