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 👍