Make all strings translatable, generate translation files.

[aschweigert]

Probably not super urgent, but if we're going to support internationalization there's still a bit of work to do, specifically:

• add a text domain
• update all of the translatable strings to use that text domain
• add translation to the build process

see: https://developer.wordpress.org/plugins/internationalization/how-to-internationalize-your-plugin/

[benlk]

A text domain was added in #47, but not all text is translatable yet, and translation is not yet part of the build process.