feat: add option to include file url

[SethFalco]

Adds the option to include a link to the references gitignore file in the README.
I prefer to add a link, so contributors know where it was derived from.

When the setting is enabled, the output looks like the following:

# https://raw.githubusercontent.com/github/gitignore/master/Maven.gitignore
target/
pom.xml.tag
pom.xml.releaseBackup
pom.xml.versionsBackup
pom.xml.next
release.properties
dependency-reduced-pom.xml
buildNumber.properties
.mvn/timing.properties
# https://github.com/takari/maven-wrapper#usage-without-binary-jar
.mvn/wrapper/maven-wrapper.jar

[CodeZombieCH]

I tought about this a few times as well, but I think your suggestion to make this optional is the best way to go.

[SethFalco]

Hey! Glad to see that you're alive! \(^-^)/

This weekend I can review all the ones you closed just to verify if they're cool, and fix the conflicts for the ones you've left open.
Thanks for peeking at the PRs!

[SethFalco]

I've rebased with master.

I also added a new script to run tests in a headless environment. This will be useful later anyway if we set up GitHub Actions, but also just more convenient.

More Info: https://code.visualstudio.com/api/working-with-extensions/continuous-integration#github-actions

[SethFalco]

I've rebased with master and resolved merge conflicts. 👍