apply the MacShellWrangler logic on linux as well?

[devoncarew]

In lib/src/process.dart, we have some logic to shell out using env variables from the user's preferred shell, when atom is run from a different shell. This helps us in situations like when the atom is run from the mac dock, and the shell process is different from the one that the user has configured with path and env variables.

We should probably look at applying this same or similar logic to linux as well (#38)