Problem: no documented way to disable logging

Solution: document --loglevel=silent and add --silent shortcut
for consistency.

(I did not see any existing regression tests for options so it's
unclear to me where to add a regression test for this... sorry.)

README.md

@@ -175,6 +175,7 @@ Command options
 | `-j n`, `--jobs n` | For `build`: run parallel builds
 | `--silly`, `--loglevel=silly` | Log all progress to console
 | `--verbose`, `--loglevel=verbose` | Log most progress to console
+| `--silent`, `--loglevel=silent` | Don't log anything
 | `--debug`          | Make Debug build (default=Release)
 
 License

lib/node-gyp.js

@@ -112,6 +112,7 @@ proto.shorthands = {
   , j: '--jobs'
   , silly: '--loglevel=silly'
   , verbose: '--loglevel=verbose'
+  , silent: '--loglevel=silent'
 }
 
 /**