Add missing space in helptext
Add initial command execution module
Add callbacks with error handling
Test that cmd.run passes child handle to callback
Replace `child_process` with `cmd`
Test .spawn is called with env and setsid
Rename .exec and .spawn to ._exec and ._spawn
Stub `child_process` functions directly
Lookup binaries from node_modules/.bin
Append '.cmd' to lookup npm binaries on windows
Update readme and changelog