No space required after WIP identifier

Fixes #2426 

Currently, the WIP: (or any other variation) prefix of a merge request title must include a trailing space for it to be recognized as a work in progress and prevent merging. 
To resolve this, I added an or pattern to the regexp to look for any one of the three possible delimiters ':', ']', or ' '. This way the trailing space isn't required but it still won't mark any titles beginning with wip as work in progress (e.g. "Wipwap").

See merge request !1876