* do initial run on full context
* decrement while loop iterations
* add context to output
* remove first param
* removing first param: change shape invariant
* models_dir argument to allow models in a separate folder
* default value for models_dir to be same as before
* allow environment variables and user home in models_dir
This fixes https://github.com/openai/gpt-2/issues/26
``` File "C:\Users\James Pollack\Desktop\gpt-2\src\encoder.py", line 112, in get_encoder
bpe_data = f.read()
File "C:\Anaconda\envs\gpt-2\lib\encodings\cp1252.py", line 23, in decode
return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 66951: character maps to <undefined>```
