attach `childProcess` to returned promise

[laggingreflex]

Lets you access the childProcess

const promise = spawn(...)
const {childProcess: cp} = promise
cp.stdout.pipe(...)
promise.then(...)

Better solution to #39 than #42

[AppVeyorBot]

:x: Build cross-spawn-promise 108 failed (commit https://github.com/zentrick/cross-spawn-promise/commit/3b095cc69e by @laggingreflex)

[AppVeyorBot]

:x: Build cross-spawn-promise 114 failed (commit https://github.com/zentrick/cross-spawn-promise/commit/9578df4b7d by @laggingreflex)

[timdp]

Thanks! :+1:

I'll hold off on releasing a new version until we've merged your other improvements as well. I might also finally add some tests.