.dojorc extends support

[JamesLMilner]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

The aim is to add a extends property to .dojorc which will allow for extension for a .dojorc from another previously defined .dojorc configuration.

This PR aims to:

• Allow for multiple nested extension, i.e. a .dojorc file can extend another .dojorc file which can extend another and so forth indefinitely.
• Provide deep 'merging' of configuration objects (i.e. configuration property values that are also objects will be recursively merged). This will only work for Objects and not Arrays or other types (we have agree this isn't such a great idea as per Feature request: allow user to merge extended arrays in tsconfig files microsoft/TypeScript#20110).

Resolves #295