Allow N_ as a decorator

[ikapelyukhin]

N_ tells gettext that the string needs translation and adds it to .po file (without substituting it with the localized string).

[highb]

👍 I wasn't aware of N_, but I see it in the API docs. Thank you for contributing! :)

[ikapelyukhin]

@highb There's also Nn_, maybe it makes sense to add it as well?

[highb]

Looks like there is also p_, s_ and then combinations of those with n_. https://github.com/mutoh/gettext I'll make an issue for it, and we can add those in a future release.