Add method that allows joining the extracted result into a string

Would it be ok if we add a method to join the extracted result into a string?

For example, usually, we want the result of a field to be a string. To get this, we often do something like this: 

```var = ''.join(sel.xpath('//div[@id="title"]/text()').extract())```

Or when we're sure that there is only one item in the list (can throw an exception if the list is empty):

```var = sel.xpath('//div[@id="title"]/text()').extract()[0]```

I think it would be more convenient if we could do something like this:

```var = sel.xpath('//div[@id="title"]/text()').join()```

Or even:

```var = sel.xpath('//div[@id="title"]/text()').join(',')```

In my opinion, this would increase the code readability.

What do you guys think of this?
```
return delimiter.join(self.extract())
```