Add shadow-select CLJS REPL type

[arichiardi]

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Thanks!

If you're just starting out to hack on CIDER you might find this section of its
manual extremely useful.

This kind of CLJS REPL is very helpful for cases when you are already watching
one or more shadow builds in shadow either after having launched the server in
a terminal or from within Cider.
This patch also refactors option normalization out of figwheel-main so that it
can be reused by every other function.

[bbatsov]

There's a bit of duplication in the two functions, but I guess I can live with this. Thanks!

Btw, a better approach might be to prompt people if they want to watch and select or simply select. That might eliminate potential confusion around having two similarly named REPL types.

[arichiardi]

Oh good idea I would just avoid to prompt twice though, so maybe some other defcustom could be useful