Allow config parameters with -Application Par Val in rebar3 shell

[peerst]

Current behaviour

Rebar3 allows the specification of a config file with the --config option of rebar shell

However it does not allow the specification of individual config key/values on the command line.

Expected behaviour

On erl command line one can specify -Application Par Val to set a config parameter in an application.

So one possibility would be to allow a similar calling option (possibly with --)

Advantage would be to be very similar to what users know from erl

Disadvantage would be the possibilities of application name with existing options.

So maybe rather go with --par Application Par Val

[ferd]

I've taken a quick look, and I think right now this is the sort of stuff that may be easier to do with the Erlang/OTP parser. I think that the getopt stuff we have has limitations: it does not appear to outright support anything but key/value formats.

We could work around this by going --par=App.Key:Val and parsing stuff out. It is non-standard and a bit odd, but it does seem that getopt supports duplicate values properly so long as we use proplists, so it could work.