Added select widget for bs4 form

[smithdc1]

@cpina re your note that the select widget isn't quite right.

[cpina]

Ha! Two days ago I did the same (with Bootstrap3 as well): cpina@544da33

I added this select:

And then somehow I realised that we had a select (even with the arrow):

But perhaps we should just add the simple select for clarity.

[smithdc1]

Let's merge your version?

I think explicit is better here, it's probably due to it not being on its own we've 'missed' it before?

[cpina]

Last night with one my fixes it initially worked with the select but not the append-select widget. Later I'll merge yours or mine. Probably mine, I think that it had one more example for BS3.