text: Title: Support "ls" command and endpoint definitions
  Body: I've been using this program for a few days now and overall I love it. I'm using it to test endpoints while developing my API.
  
  One issue I have however is that whenever I type `cd`, my muscle memory kicks in and I immediately follow with an `ls` out of habit. It's always a little disappointing when this doesn't work :( I was thinking of ways to solve this, and it would be great if I could pass in my endpoint definition file ala
  
  ``` bash
  $ http-prompt --specification=my_api_spec.json
  ```
  
  This would enable http-prompt to do a bunch of smart things like suggest endpoints when I type `ls` or  autocomplete parameter names. Could also add a `help` command to view the documentation for any given endpoint. There are a couple of widely used spec formats, with Swagger probably being the most popular
  
  http://nordicapis.com/top-specification-formats-for-rest-apis/
  