Update Code/Helpers/MarkdownTextViewManipulation.m

 In method - (void)pushTitleLevelOnSelectedRange, if selectedRange.location 
is the start of the selected line, beforeTextSelection will have no content, 
and [beforeTextSelection substringWithRange:NSMakeRange(0, 1)] throw exception
Merge pull request #1 from yebing115/master

Method - (void)pushTitleLevelOnSelectedRange will sometimes throw exception when press the "#" button in editing note