feat: Add show option to the certs plugin

The 'server.key' and 'server.crt' can printed to stdin.
The two options are:
    `dokku certs:show <app> key > server.key`
    `dokku certs:show <app> crt > server.crt`
This will create two files, `server.key` and `server.crt`.
Those files contain the private key and the certificate.

Includes tests.

plugins/certs/help-functions

@@ -34,6 +34,7 @@ fn-help-content() {
     certs:remove <app>, Remove an SSL Endpoint from an app
     certs:report [<app>] [<flag>], Displays an ssl report for one or more apps
     certs:rollback <app>, [NOT IMPLEMENTED] Rollback an SSL Endpoint for an app
+    certs:show <app> <crt|key>, Show the server.crt or server.key on stdout
     certs:update <app> CRT KEY, Update an SSL Endpoint on an app. Can also import from a tarball on stdin
 help_content
 }