Append custom env vars instead of replace in Process (#168, #169)
PR #162 refactored the default configuration code. This
introduced a subtle bug where `env` became `[]string{}` instead of the
default of `nil`.
In golang, `exec.Cmd.Env == nil` means to use the "current process's
environment". By setting it to `[]string{}` as a default the Process's
environment was emptied out which caused an array of strange and
difficult to troubleshoot behaviour. See issues #168 and #169
This commit changes the behaviour to append model configured environment
variables to the default list rather than replace them.
49035e2e8e