Profile support

[ArtemGet]

I would like to add support for profiles:
src\resources\application.yaml

profile: {MY_PROFILE:local}
db:
  url: {DB_URL}
  login: {DB_LOGIN}
  pass: {DB_PASS}

src\resources\application-local.yaml

db:
  url: my URL for local development  
  login: my login for local development  
  pass: my password for local development  

The idea is that when retrieving a parameter, we take the value not from the main config file but from the file specified in the profile.

Currently, to achieve this functionality, we'd have to write something like:

String dbUrl = new EVal(
    "db.url",
    String.format(
        "src/resources/application-%s.yaml",
        new EVal("profile").value()
    )
).value();

In general, things could get even worse because here we can't tell whether a profile exists or if the user forgot to specify its value (or if the value isn't set in the environment).

Ideally, I'd like to see something like:

String dbUrl = new EValProf("db.url").value();

or

String dbUrl = new EValProf("db.url", "path_to_application.yaml").value();

And, of course, error handling needs to be taken care of:

• If the profile attribute is missing in application.yaml, simply fetch the attribute as usual—from application.yaml.
• If the profile attribute exists but its value is empty (or if it references an environment variable with no default value and that variable is empty), throw a clear error to the user when value() is called.
• If the profile attribute exists, reuse the EVal class with the new path (as described at the beginning) and retrieve the desired value from the appropriate YAML file.