Allow muting certain upgrade messages using the $OMZ_UPGRADE_TEXT variable

The upgrade script now determines how verbose it should be based on `$OMZ_UPGRADE_TEXT`. You can think of that variable as sort of a log level for the upgrade script. These are the possible values:

* default: print everything (same as before)
* `noasciiart`: print everything except the huge ASCII art saying “oh my zsh”
* `error`: only print the error message if the update fails
* `none`: print nothing