Add possibility to pass `env` variables, like System.cmd

[philipgiuliani]

System.cmd has the possibility to pass a keyword list of env variables for executing the command.

See: https://github.com/elixir-lang/elixir/blob/v1.10.2/lib/elixir/lib/system.ex#L730

It would be nice to also have this feature in Rambo!

[jayjun]

System.cmd accepts a list of string-string tuples, not a keyword list.

iex> Keyword.keyword?([{"MIX_ENV", "test"}])
false

Good news is, Rambo already accepts that and it‘s documented!

[philipgiuliani]

Oh, I looked for it but missed it 😅 Thank you!