MXS-1300: Add simple diff calculation

The `cluster diff` command prints out the difference between the source
and target MaxScale. This allows users to detect changes that have
happened on one MaxScale but not on the other.

maxctrl/lib/core.js

@@ -78,6 +78,7 @@ program
     .command(require('./alter.js'))
     .command(require('./rotate.js'))
     .command(require('./call.js'))
+    .command(require('./cluster.js'))
     .help()
     .demandCommand(1, 'At least one command is required')
     .command('*', 'the default command', {}, function(argv) {