Feature: Sync version

[dlmr]

Hi!

Would a PR that adds a sync variant of spawn-command be accepted?

This could be done in mainly two ways as I see it.

A.

Provide an alternative sync version and export both of them.

var spawnCommand = require('spawn-command');

spawnCommand(); // As before
spawnCommand.sync(); // A new sync variant

B.

Support an optional third argument that takes the function that should be used as spawn.

var spawnSync = require('child_process').spawnSync;
var spawnCommand = require('spawn-command');

spawnCommand(command, options, spawnSync);