rename text domain

[benlk]

When did I introduce the text domain? Can we rename it from pym-shortcode to pym-embeds without affecting stuff?

I mean, this plugin doesn't ship with translation files at the moment, so how hard could it be?

[benlk]

Text domain was added in #34, so it's new in 1.3.2.1 and we can change it before 1.3.2.1 is released.