Allow for arrays of commands

[Renari]

It would be nice if instead of:

{
  "@semantic-release/exec",
  {
    "prepareCmd":  "sed -i 's/<version>/${nextRelease.version}/' package.json && npm run build && tar -czf my.tar.gz * && zip my.zip *"
  }
}

You could specify an array of commands instead:

{
  "@semantic-release/exec",
  {
    "prepareCmd":  [
      "sed -i 's/<version>/${nextRelease.version}/' package.json",
      "npm run build",
      "tar -czf my.tar.gz *",
      "zip my.zip *"
    ]
  }
}