Add ability to extend default options

[JasonWeinzierl]

There are several rules with options that accept multiple values. Adding items to these sets requires overwriting the entire set, so if you want to keep the existing defaults and simple add a few items, you must copy-paste the entire set into your config. This is inconvenient and also requires maintenance as we add items to the default sets in the future.

Introduce a new mode for extending a default set, as well as keeping existing behavior of overwriting.

Affected rules and their options with a default set:

• no-unsafe-takeuntil: the allow option
• no-ignored-subscription: the completers and postCompleters options (added in feat(no-ignored-subscription): ignore for certain operators #187)
• finnish: the names and types options
• sufix-subjects: types

Consider an API like the following:

    allowedStrings?: {
      mode?: "extend" | "override";
      values?: string[];
    } | string[]; // Default to "override" if array of strings.

Semi-related: #51