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