Add the ability to manually execute checks against an application

Checks can be run in several ways:

- checks:run APP
- checks:run APP PROCESS_TYPE
- checks:run APP PROCESS_TYPE.CONTAINER_INDEX

This will reuse the existing checks infrastructure as defined by the checks plugin, and therefore if no checks file is defined, it will default to the normal Dokku container check.

Closes #2218

plugins/checks/commands

@@ -10,6 +10,7 @@ case "$1" in
     checks <app>, Show zero-downtime status
     checks:disable <app> [process-type(s)], Disable zero-downtime deployment for all processes (or comma-separated process-type list) ***WARNING: this will cause downtime during deployments***
     checks:enable <app> [process-type(s)], Enable zero-downtime deployment for all processes (or comma-separated process-type list)
+    checks:run <app> [process-type(s)], Runs zero-downtime checks for all processes (or comma-separated process-type list)
     checks:skip <app> [process-type(s)], Skip zero-downtime checks for all processes (or comma-separated process-type list)
 help_content
     }