bug#21316: 25.0.50; eww-browse-with-external-browser should respect browse-url by default

[Artur Malabarba]

The browser used by `eww-browse-with-external-browser' is given by the
variable `shr-external-browser'. The default value of this variable is
`browse-url-default-browser', which gives me the impression that it's
trying to fallback on whatever `browse-url' would do.
But `browse-url' behaviour is actually configured by
`browse-url-browser-function'

In terms of actual user experience, I was surprised when
`eww-browse-with-external-browser' launched the wrong browser for me.

In my opinion:
- There should be a value for this variable (nil?) that simply
fallsback on browse-url. (pretty straightforward)
- This value should be the default. (slightly subjective)