Ability to not force all external links to open in new browser by default.

[keithmifsud]

The User can input an entry in the Jekyll's config file to state which links should open in a new browser by specifying a CSS class name. Like so:

target-blank:
  css_class: ext-link

If a css_class is specified, only links containing that class name will open in a new browser unless overwritten by another tag of course 😄

• Refactor the main class by using the initialised @config instance variable.

• Refactor tests by removing unit tests to private methods.

• Enforce standards using rubocop

• Comment the code.

• Update the Readme.md file to reflect this feature.

• Merge into the pre-release branch.