Enhance the gettext support to recognize more macros

Indent has recently acquired a nice feature: it doesn't break _("...")
and N_("..."), when the '--gettext-strings' option is used.

However, some projects use additional macros, like Q_("..."), as in:

https://developer.gnome.org/glib/stable/glib-I18N.html

I myself use E_("...") to mark messages intended for programmers
instead of end-users (I treat it as a no-op but a future maintainer
may choose to treat it like _("...").)

So, I was wondering whether we could solve this problem simply by
making indent recognize the token X_ instead of N_, where X is any
uppercase letter. What are your thoughts on this?