Add support for naming env variables

[faide]

Hi, I added support for a new config variable that enables the user to name the environment variable its program will receive.

It is especially useful for source=string and for source=entire-payload schemes

[
  {
    "id": "push",
    "execute-command": "bb2mm",
    "command-working-directory": "/tmp",
    "pass-environment-to-command":
    [
      {
        "source": "entire-payload",
        "envname": "PAYLOAD"
      },
      {
        "source": "string",
        "name": "PUSH"
      },
      {
        "source": "string",
        "envname": "MMURL",
        "name": "https://someurl/hooks/somehooktokengeneratedbymm"
      }
    ]
  }
]

will give you nice env variables:

• HOOK_PAYLOAD (using the introduced feature)
• HOOK_MMURL (using the introduced feature)
• HOOK_PUSH

without this feature the obtained vars are:

• HOOK_ (it has no name and thus is not named :/ )
• HOOK_https://someurl/hooks/somehooktokengeneratedbymm (the name is also the content and thus you won't be able to find this variable without knowing its content first, which kind of defeats the purpose)

[adnanh]

Thank you very much for your contribution and making webhook a better tool. :-)