add keep_serving option to webserv command

[thorinaboenke]

this PR relates to issue #186

• adds an option keep_serving to webserv command to keep serving the file after the first request

how to test:

commands:
  - type: webserv
    local_path: "/path/to/your/file"
    port: 8000
    background: True
    keep_serving: True

  - type: sleep
    seconds: 60

perform several curl requests curl <AttackeMateIP>:8000 during the sleep command