Feature request: Convert to unescaped string

When urls (or parts thereof) are converted to a string, they're always escaped:

```python
>>> url = furl('foo.bar/fire truck?hello world=#hi there')
>>> str(url)
'foo.bar/fire%20truck?hello+world=#hi%20there'
>>> str(url.path)
'foo.bar/fire%20truck'
>>> str(url.query)
'hello+world='
>>> str(url.fragment)
'hi%20there'
```

It would be useful if there was a way to obtain unescaped strings:

```python
>>> url.unescaped_str()
'foo.bar/fire truck?hello world=#hi there'
>>> url.path.unescaped_str()
'foo.bar/fire truck'
>>> url.query.unescaped_str()
'hello world='
>>> url.fragment.unescaped_str()
'hi there'
```