Close streaming output channels

[daniel-nichter]

• Close streaming output chans (cmd.Stdout, cmd.Stderr) (issue channel stderr and stdout are not closed #26)
• Refactor NewCmd
• Update blocking-streaming example
• Test on go1.12 and 1.13

[coveralls]

Coverage decreased (-1.3%) to 98.684% when pulling fac9413 on close-streaming-chans into 1e96b7e on master.