feat: Add selected element in Find result

[Turom]

Add the Agouti selected element in the find result, for example in a result.findElement property so the element content or attributes may be tested in Venom assertions.

https://github.com/runabove/venom/blob/0ae13f092534bc1f03c14b576846ff4ff5f030d5/executors/web/web.go#L136

Looks like the Agouti Selection obtained can return elements with
https://github.com/sclevine/agouti/blob/dfc6680bb9fcb3f3d3b6f506a1d1e77fbc9c9613/selection.go#L53

Some element properties maybe exposed in result.findElement
http://godoc.org/github.com/sclevine/agouti/api#Element

[fehrnah]

Looking at the code, it seems that this exists :
In the executor result:

venom/executors/web/web.go

Line 52 in 80cff7a

Text string `json:"text,omitempty" yaml:"text,omitempty"`

In the find action:

venom/executors/web/web.go

Line 220 in 80cff7a

r.Text = text

But it is not documented anywhere (and not tested either)

[yesnault]

web executor was refactored on 1.2.0-beta.1 version. Feel free to reopen if needed.