Enable headless template validation only when "-headless" option is used

[ehsandeep]

Please describe your feature request:

As of now, when nuclei -validate or nuclei -t test.yaml -validate is used, nuclei start to download headless browser in the background even though the headless browser is not requested or required to validate given template.

current behavior should be only executed when -headless flag is used which specifically mentions use headless engine.

Describe the use case of this feature:

• Will avoid downloading headless browsers when it is not required.