Add optional parameter to 'resign' command, allowing to set 'appId'

'briar resign' now accepts an optional parameter to set a new application id (CFBundleIdentifier) when resigning.
Following unix command line conventions, this should be:

```
briar resign </path/to/your.ipa> </path/to/your.mobileprovision> <wildcard-prefix> <signing-identity> [optional-application-id]
```