Synchronize tags

synchronizing tags works, except removing tags from VW to TW is not yet
implemented
remove tags from TW which are not in VW anymore
notify of tasks which are not found anymore in TW
Mark deleted tasks as deleted and don't update them anymore
Use curly braces to retrieve config variables
configuration option to specify additional args for task

This can be useful for example to set an alternative .taskrc location to
keep the vimwiki tasks in a separate file or for testing purposes to
keep the main task files clean