Make CommandLineConfiguration mutable

[adamsitnik]

• Add Symbol.Parse, make Parser static #2054 needs to be implemented first, as it's not clear whether we will be able to remove the CommandLineBuilder type. Builder types are the only types in BCL that are allowed to use fluent APIs (returning reference to this to allow for calls chaining) and I am currently not sure if we are going to be able to repsent all configuration options as properties (many extension methods set two things at a time).
• Introduce Directive symbol type #2056 needs to be implemented first as it will have great impact on how Directives will be configured. We don't want to have EnableDirectiveXYZ property for every Directive
• make all the internal fields and properties public, with nice names
• make all the public properties mutable