Add translation string "today_format" to POT.

[yvesvanlaer]

Great plugin, awesome work!

I noticed that the "today_format" string is not available in the translation POT.
It is accessable because it has been added like all the other translated strings (see IsOpen.php).
'today_format' => __('Opening Hours today: %1$s', 'wp-opening-hours'),
So I added it manually to the translation POT.

#: classes/OpeningHours/Module/Shortcode/IsOpen.php:32
#, php-format
msgid "Opening Hours today: %1$s"
msgstr ""

But please add this to the master so I don't lose this after an update.

Thank you very much!