Allow Gaps Between CLOCK splits

[justintaft]

Create an option for inserting a gap when splitting records. E.g., a clock entry from 9:00 to 14:00 could be split into one from 9:00 to 12:00 and then one from 12:30 to 14:00. This would help those who start some task in the morning, but forget to clock out when going to lunch.

Idea by majestic_blueberry
https://www.reddit.com/r/emacs/comments/9bs3mo/split_orgmode_clock_entry_into_two_entries_elisp/e55jq84/