Improve setting date format

[DHaussermann]

The way the date format is set is too brittle. Setting an incorrect format or, setting it in a way that is correct with a date other than January 2nd will cause the plugin to behave incorrectly in various ways.

Some suggestions form @marianunez on the previous PR about how this can be improved...

• Limit it to a pre-set list of date formats like you suggest but not sure how this would work on placing the rest of your hashtag text (just before the date?).
• Design a different way/UI to define your date format all together

My Suggestion would be to have a textbox where the hash-tag can be set and a radio button to select your desired date format.