[New Feature Proposal] Add file serving capabilities.

# File Serving

## Use Case

As a developer, I often want to share services/pages/etc running on my development machine with the outside world. ngrok is great for this.

However, I also sometimes want to expose plain HTML/documents/files to the outside world without running a webserver -- often I don't know how to start a webserver. A great example is the web designers with whom I work; they want to share awesome HTML/CSS/JS mockups with me but aren't using Node/Nginx/etc and have no way to show me these files.

## Workaround

Often, I've just used something like `python -m SimpleHTTPServer 9000` to share a directory and then use `ngrok 9000`. This works, but requires me to use Python (which I may or may not have installed) and uses Python's "test" server which is slow, among other things. Other alternatives include Nginx (and all the setup/requirements) and http-server (which requires Node and all the setup associated).

## Solution

Why not leverage Go's powerful `net/http` server? It is performant and simple to implement. It also doesn't require any third party software (no Python or Node server).

## My Approach

1. Added two command line options: `serve` (a bool) and `serveDir` (a string)
2. Add these options to the configuration file (see below)
3. Allow the command line to override any configuration (or lack of) options
4. In the controller, spin up a web server for each unique tunnel (on a unique port). Use `http.FileServer` to serve a directory.

## Questions to be answered/Work to be done

1. I don't know how to update https://ngrok.com/usage to include documentation
2. I believe I've covered all cases but I'm not sure how this plays out when you aren't using `http` or `https` protocol
3. I don't know if this fits in with your project vision.

## Configuration

My `.ngrok` file looks like this:

```
tunnels:
  nickp:
    proto:
      https: 9000

  share:
    serve: true
    proto:
      https: 9001

  mockups:
    serve: true
    serveDir: /Users/nickp/mockups
    proto:
      https: 9010
```

So my workflow consists of running `ngrok start share` which I have aliased to just `share` to share my current directory.
When I want to share some mockup designs or something, I just run `ngrok start mockups` and all is well. Ideally, this would be what a designer (or related) does when they want to share their work.

## Next Steps

1. Please let me know if this proposal makes sense
2. Let me know how the code looks and how we can work together to get this merged
3. If it is possible, how to update the usage docs on ngrok.com so others will know how to use this.