Add Support for EXPRESS_PORT Variable

As discussed in #728, this PR adds support for an `EXPRESS_PORT` variable to set the port for Express to listen on while also retaining support for the current `PORT` variable. `EXPRESS_PORT` will be favored over `PORT`. My only outstanding question is whether we should also add `EXPRESS_BIND_ADDRESS` - this hasn't been an issue I've run into, but it might be nice for the sake of consistency.

cc: @technicalpickles