Add separator option

For making easier to read each tweets.

Usage:

```ruby
Earthquake.config[:separator] = "-"
```

Output example:

```
[$aa] joker1007: tweet1 29 Jul 17:09 - earthquake
------------------------------------------------
[$ab] joker1007: tweet2 29 Jul 17:09 - earthquake
------------------------------------------------
[$ac] joker1007: tweet3 29 Jul 17:09 - earthquake
------------------------------------------------
```