commands: add streaming output

[BrianHicks]

Looks like this:

converge > ./converge apply samples/basic.hcl
INFO[0000] checking status                               filename=samples/basic.hcl path=basic.hcl/render
INFO[0000] applying                                      filename=samples/basic.hcl path=basic.hcl/render

basic.hcl/render:
    Status: "cat: test.txt: No such file or directory\n" => "Hello, World!\n"
    Success: true

Closes #34