Support custom options

[TheMulti0]

Following #4, this PR implements the support of passing custom IOptions to YoutubeDl.

This PR includes the following changes:

• [x] IOption is now public and has the bool IsCustom { get; } property.
• [x] Option includes an overload constructor which takes bool isCustom as an argument.
• [x] OptionSet.Custom.cs adds the IOption[] CustomOptions { get; set; } property to OptionSet.
• [x] OptionSet.GetOptionFlags includes the custom options.
• [x] OptionSet.GetOptions has been renamed to GetKnownOptions.
• [x] OptionSet.OverrideOptions combines the custom options of both old and new OptionSet using a custom IOption comparer (OptionComparer) which uses string comparison.
• [x] OptionSet.FromString parses unknown options as custom options. If the option contains a value (for example: "--my-option value") it is considered an IOption<string>. If not (for example: "--my-option"), it is considered an IOption<bool>.

[TheMulti0]

Looks great! Thanks a lot for your contribution, really appreciate it!

Happy to help, please don't forget to update the package on NuGet! 😋