Added support for runtime options

[Niteshvgupta]

The purpose of this commit is to allow support for access to set and get run-time options. Extensions of Monolog, such as Cascade, will need to set a run-time options from a config file and access it after parsing and losing access to the config file.

[PhiloEpisteme]

It is safer to do array_key_exists($name, static::$options) in case someone sets a value as null for some reason. Also, when accessing static member variables and properties, prefer static:: to self::. The reason is that static supports late static binding whereas self does not.

[PhiloEpisteme]

Can you explain what the purpose of these run-time options is? One use case we need is to change behavior when we ask Cascade for a logger that does not exist. It feels like that work can be in Cascade and not in Monolog. For example, Cascade could ask the Monolog::Registry for a logger, if it doesn't exist, ask for the first parent, if it doesn't exist ask for the next parent etc until one is found.