Make config file parsers pluggable (#1)

In order to simplify deps, this PR makes config file formats pluggable. The extensions supported by default are "json" and "eterm".
The default json decoder is the one from OTP, but if one prefers another (e.g. `zj`), it's possible to choose that when installing the gmconfig env.

Example of adding a yaml decoder:
```erlang
instrument_gmconfig() ->
    gmconfig:set_gmconfig_env(gmconfig_env()).

-spec gmconfig_env() -> gmconfig:gmconfig().
gmconfig_env() ->
    #{ ...
     , config_formats => add_yamerl() }.

add_yamerl() ->
    Default = maps:get(config_formats, gmconfig:default_gmconfig_env()),
    case maps:is_key("yaml", Default) of
        true ->
            Default;
        false ->
            Default#{"yaml" => fun yamerl_decode/1}
    end.

yamerl_decode(Bin) ->
    yamerl:decode(Bin, [{str_node_as_binary, true},
                        {map_node_format, map}]).
```

Co-authored-by: Ulf Wiger <ulf@wiger.net>
Reviewed-on: #1

rebar.config

@@ -3,9 +3,7 @@
 {plugins, [rebar3_hex]}.
 
 {deps, [
-         {zj, {git, "https://gitlab.com/zxq9/zj.git", {tag, "1.1.2"}}}
-       , {yamerl, "0.10.0"}
-       , {setup, "2.1.2"}
+        {setup, "2.2.1"}
        ]}.
 
 {profiles, [
@@ -17,5 +15,5 @@
                deprecated_function_calls, deprecated_functions]}.
 
 {dialyzer, [ {warnings, [unknown]}
-           , {base_plt_apps, [erts, kernel, stdlib, yamerl, zj, setup]}
+           , {base_plt_apps, [erts, kernel, stdlib, setup]}
            ]}.