Remote template workflow lists

[EndPositive]

This PR closes #970 by adding the flags template-url and -workflow-url which retrieve lists of templates from remote URLs and add them to the corresponding Nuclei run. If a URL is not valid or it gives an unexpected response status, Nuclei exits. Only HTTP supported.

TEMPLATES:
   -tl                           list all available templates
   -t, -templates string[]       template or template directory paths to include in the scan
   -tu, -template-urls string[]  URL to a list of templates
   -w, -workflows string[]       list of workflows to run
   -wu, -workflow-urls string[]  URL to a list of workflows to run
   -nt, -new-templates           run newly added templates only
   -validate                     validate the passed templates to nuclei

[Ice3man543]

@EndPositive nice work on the PR. Can you fix the lint errors please?

[Ice3man543]

lgtm!