Add --list option, and prompt user to open an issue

Hi @jlord how is this.

```
$ offline-issues --list

You have these issues:
  1. github-github#1
  2. github-github#2
  3. github-github#3
  4. github-github#4
  5. muan-emoji#1
  6. muan-github-gmail#1

Which issue do you want to open? [1-6] sup
error:   Invalid input for Which issue do you want to open? [1-6]
error:   Must be a number and <=6
Which issue do you want to open? [1-6] 3
Opening ./html/github-github-3.html
```

I really want issue titles though.
Need to handle the `err` here:

```js
if (err) return cb(err, "Error reading directory")
```