Add add_words() function

[justinchuntingho]

Let users define their own words

[justinchuntingho]

it should accept: string, character vector, and text file (one word per line).

[justinchuntingho]

approach:

1. assign a hidden worker
2. use JiebaR functionalities to add word to the worker

[justinchuntingho]

Added the function, which accepts string and char vector. Still need to expand to detect text file