Better string output

Actually, string that in origin was like
```yaml
a: >
  some
  inline
  content
```

Will be output as:
```yaml
a: |
  some inline content
```

Instead with this edit, will be output accordingly with `line_width` option:
```yaml
# line_width = 100
a: some inline content

# line_width = 11
a: >
  some inline
  content
```

It's not that big of a change, but I think it's nice to have, especially for meticulous people like me :)
This looks a little ugly, what about:

```ruby
elsif str.length > line_width
```

```ruby
def line_width
  @options[:line_width] || Float::INFINITY
end
```