Add `optionalValues` and `requiredValues` options

[rigwild]

Closes #41, resolves #45

First, thank you for this lib, I use it on almost every projects of mine! 😊

This PR adds the options optionalValues and requiredValues. I thought it would be better to not allow allowEmptyValues to be a Set just like in #41, and make it new separate options.

How it works

• Uses Set as mentioned in #41
• optionalValues: Values in optionalValues will not be required anymore
• allowEmptyValues: true + requiredValues: All values are optional except ones in requiredValues

Errors

• allowEmptyValues: true + optionalValues: Error, useless as all values are already optional
• allowEmptyValues: false + requiredValues: Error, everything is already required
• optionalValues or requiredValues are not Set: Error, must be Set

How to use

// Any value can be empty except `THIS_ENV_IS_REQUIRED_ANYWAY` and `THIS_ONE_TOO`
require('dotenv-safe').config({
    allowEmptyValues: true,
    requiredValues: new Set(['THIS_ENV_IS_REQUIRED_ANYWAY', 'THIS_ONE_TOO']),
    example: './.my-env-example-filename'
});

// Every values are required except `THIS_ENV_IS_OPTIONAL` and `THIS_ONE_TOO`
require('dotenv-safe').config({
    optionalValues: new Set(['THIS_ENV_IS_OPTIONAL', 'THIS_ONE_TOO']),
    example: './.my-env-example-filename'
});

Let me know what you think of it 😄