Update plugins_disabled/omnifocus.rb

2 changes:

1. Ensure that only valid characters are saved from omnifocus.  
If a completed task contains a bullet, for example, the dayone.rb 
script will fail at content.scan on line 6 with the following error:
in `scan': invalid byte sequence in US-ASCII (ArgumentError)


2. Added the ability to show the number of completed tasks for the day in the output file.