named argument in `capture` should be used as the capture name.

So instead of

``` r
rex(capture(name = 'test', anything))
```

You can write

``` r
rex(capture(test = anything))
```

Or the short form

``` r
rex(.(test = anything))
```

It should not require a huge change except for the `capture` implementation.

I am undecided whether to try and maintain backwards compatibility with the previous behavior or not.
