Do not start packages on `parts init -`, only if --start option given

The command `eval $(parts init -)` was previously encouraged and
used in docs, causing stray output from packages starting to be
evaluated.  We now require an explicit --start option to run
package starts.
Merge pull request #106 from nitrous-io/add_start_option_to_init

Do not start packages on `parts init -`, only if --start option given