updating the localization functions to that `load_textdomain` is called.

Removing `load_plugin_textdomain` and leaving only `load_textdomain`.

This is standard as per new WordPress practices[0].

Ultimately, it makes language files more portable as they are still accessible via WordPress even if the plugin developer did not include them with the plugin itself.

Related #120

[0] https://github.com/tommcfarlin/WordPress-Plugin-Boilerplate/commit/d2eb67079b3dd8db76185737a72b5d2efda542ae#commitcomment-4644357

Signed-off-by: Tom McFarlin <tom@tommcfarlin.com>