Fill missing settings with default values

[gazben]

Currently if you deploy a new version to a server and the migrations are still running you get a MissingSettings exception. For long running migrations and big deployments this can be a problem.

This proposal adds a fallback option to settings defined in the Settings object. If you set the default in the code the settings mapper will return that default value instead of throwing an exception. For nullable values the default will be null without changing anything in the Settings object.

Example:

    // Returns false without migrations
    public bool $mySetting = false;
    // Returns null
    public ?string $myOtherSetting;
    // Fails
    public string $myOtherSetting;

What do you think about this proposal?

Credits to @andrasszommer

[LarsWiegers]

Love this and definitely a good idea

[mrmmg]

YES, I upvote this PR!

After 3 hours of debugging this issue, I opened issue #307.

Please merge this PR into package!