Allow use of yamerl without starting the app

[artman41]

Forces an application load prior to the application:get_env/2

[coveralls]

coverage: 77.575% (+0.01%) from 77.564%
when pulling 4722d1e on artman41:patch-1
into a24f448 on yakaz:master.

[dumbbell]

Hi! Thank you for the contribution!

application:load/1 is a synchronous call to the application controller, thus it can be expensive.

I see two possible approaches:

1. Load the application in e.g. yamerl_constr:setup_node/2 which semantically is supposed to be called once unlike yamerl_app:get_param/1. We can also check if the application is already loaded before with application:loaded_application/0 which is a simple ETS read.
2. Be optimistic in yamerl_app:get_param/1 and call application:get_env/2 in a case and if it returns undefined, load the application and call application:get_env/2 again.

What do you think? I have a slight preference for option 1.

[artman41]

since the app resource defines the default env as {node_mods, []}, I'd be tempted to just swap the application:env_env/2 to an application:get_env/3 where it would default to an empty list

Otherwise, having a forced load occur in the setup would be a good middleground 👍

[dumbbell]

You are right this is a better idea. Would you mind modifying your pull request to use application:get_env/3?

[artman41]

@dumbbell have a look and let me know what you think 👍

My go-to style is to have a config module that'd look something like this

-module(yamerl_config).

-export([
    get_node_mods/0,
    set_node_mods/1
]).

-spec get_node_mods() -> list(module()).
get_node_mods() ->
    get_value(node_mods, []).

-spec set_node_mods(list(module())) -> ok.
set_node_mods(Value) ->
    set_value(node_mods, Value).
    
%% Internal Functions

get_value(Key) ->
    application:get_env(yamerl, Key).
    
get_value(Key, Default) ->
    case get_value(Key) of
        undefined ->
            Default;
        {ok, Value} ->
            Value
    end. 
    
set_value(Key, Value) ->
    application:set_env(yamerl, Key, Value).

But I've tried to fit with the existing style 😄

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[dumbbell]

For very simple parameter handling like here, I usually put that in the *_app.erl module.

I like your suggestion to use dedicated functions like get_node_mods(). Don’t hesitate to apply that to this pull request if you like :)

Otherwise, please squash all commits into one.