function in jsngram.json2 module
Description
Put an end bracket to a json file. The file should not have any end brackets. It appends without looking the file to get performance.
This appends a new line for the end bracket. 
has_end function depends on this feature. 
      
Example
jsngram.json2.json_append('/scratch/a.json', 1)
jsngram.json2.json_append('/scratch/a.json', 2)
jsngram.json2.json_end('/scratch/a.json')
# [1,2]
      
    Usage
jsngram.json2.json_end(file_name)
Argument file_name must be a full path to a json file.