Add ability to override --delete-removed flag

If the parameter is not set, then previous behavior will be respected.
So the `delete-removed` flag will be added. If the parameter is set,
then it needs to be `yes`, otherwise it will be skipped.
Update README and version
Merge pull request #6 from wercker/feature/SupportOptionalFlags

Add ability to override --delete-removed flag
warn instead of warning
Update wercker-step.yml

version bump as well
Merge pull request #7 from wercker/warn

warn instead of warning
Merge branch master of wercker/step-s3sync

Conflicts:
	wercker-step.yml

Some problems occurred with the current version of the step. Updating to
the latest version of the step to see if that fixes it.
Bump version to 1.2.1

Bumped the version to 1.2.1. Keeping this somewhat in sync with
Wercker's step. Unfortunately, we must bump the version number to get
Wercker to pick up the changes. There's not a lot of logic to the
version number otherwise.