Disabling autocomplete in string and comment contexts

Fix for MAGN-5099: http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5099

Will add test cases shortly.

@Benglin please review
I'm puzzled by this, how can one possibly know if anything is inside a comment or string, if it's just given a ```string```? I do believe we need to take a step back and give ```CodeCompletionParser``` a careful thought it deserves. The following ```ParseContext``` has the traits of mistakes that we made before. I am definitely not one to talk about parser, but I do recognize the pattern that led to my previous mistakes. I start to believe that we need a full fledged parser now that we start doing string manipulation... can you seek advice from experts in the area? :)