Refactor prompts (#115)

docs/how_to/custom_prompts.md

docs/reference/prompts.rst

examples/paul_graham_essay/TestEssay.ipynb

examples/paul_graham_essay/index_with_query.json

gpt_index/__init__.py

gpt_index/indices/keyword_table/base.py

gpt_index/indices/list/base.py

gpt_index/indices/query/keyword_table/query.py

gpt_index/indices/query/list/embedding_query.py

gpt_index/indices/query/list/query.py

gpt_index/indices/query/tree/embedding_query.py

gpt_index/indices/query/tree/leaf_query.py

gpt_index/indices/query/tree/retrieve_query.py

gpt_index/indices/query/vector_store/faiss.py

gpt_index/indices/tree/base.py

gpt_index/indices/vector_store/faiss.py

gpt_index/langchain_helpers/chain_wrapper.py

gpt_index/prompts/base.py

gpt_index/prompts/default_prompts.py

gpt_index/prompts/prompt_type.py