Make @help doc dynamic

[renxinhe]

For scalability, the @help command should generate its contents dynamically from DOCS.md.

Example format:
User:

@help

Bot:

List of avaliable commands:

@color
@meme
@weather

User:

@help color

Bot:

Color
https://github.com/renxinhe/facebook-chat-bot/blob/master/DOCS.md#color

Description
This trigger can set the color of a chat thread.

Usage
@color <RGB Hex>

Arguments
RGB Hex: A six digit hexadecimal number representing the target color. Both upper and lower cases are supported.