Set a starting date

The proposed commit add an optional textfield in the configuration:

• if set: the month shown at the first calendar load will be the one set in the texfield,
• if left empty: it will show the current date month

furthermore the weekdays' names array in the .js were surrounded by Drupal.t() to make them available for translation

Thank you @d.sibaud for your feedback and MR.
I tested your code but didn't work as expected on my side.
Ended up also adding the end date.
I'll close the MR since the funcionality is already available in the latest release,

rfmarcelino → closed merge request !1