Datacenter flag is not supported when firing an event using the cli

#### Overview of the Issue

According to the documentation (https://www.consul.io/docs/commands/event.html#datacenter-lt-name-gt-) the `-datacenter` and `-stale` flags are supported by the command line. But when using version 1.4.4 they are not available through the command line but are through the HTTP API (https://www.consul.io/api/event.html#dc).

Also looking at the current code it appears to not be available..

The documentation should be updated to remove the not supported flags, and/or the cli should be updated to support the flags same as the HTTP API