Add pipe input for prompt on gpt examples (#38)

Add pipe input for prompt on gpt examples (#38)

Enable prompt input through pipe, instead of using -p option.
This makes easier to give longer and multiple lines for the prompt.

Test:
 $ echo "This is an example" > prompt.txt
 $ cat prompt.txt | ./bin/gpt-j -m models/gpt-j-6B/ggml-model.bin
 $ cat promot.txt | ./bin/gpt-2 -m models/gpt-2-117M/ggml-model.bin

Note that -p option and no -p specified case are kept.
 $ ./bin/gpt-j -m models/gpt-j-6B/ggml-model.bin -p "This is an example"
 $ ./bin/gpt-j -m models/gpt-j-6B/ggml-model.bin
 $ ./bin/gpt-2 -m models/gpt-2-117M/ggml-model.bin -p "This is an example"
 $ ./bin/gpt-2 -m models/gpt-2-117M/ggml-model.bin