Allow adding or removing individual environment variables.

Specifying an environment string as +KEY=VALUE will append KEY=VALUE to
the existing block, or create a new one if none was previously
configured.  If KEY is already present, +KEY=VALUE will override it in
place.

Specifying -KEY=VALUE will remove KEY=VALUE from the existing block.

Specifying -KEY will remove KEY regardless of its value.

Removing the last key will delete the whole block.

Specifying :KEY=VALUE is equivalent to KEY=VALUE, ie it creates a new
block with only KEY=VALUE present.  Its main purpose is to make scripts
easier to read.

    nssm set <servicename> AppEnvironment :FIRST=one
    nssm set <servicename> AppEnvironment +SECOND=two
    nssm set <servicename> AppEnvironment +THIRD=two