feat(@angular/cli): provide '--sourcemaps' alias for build (#4462)

Author: clydin

packages/@angular/cli/commands/build.ts

@@ -14,7 +14,7 @@ export const BaseBuildCommandOptions: any = [
   { name: 'environment', type: String, aliases: ['e'] },
   { name: 'output-path', type: 'Path', aliases: ['op'] },
   { name: 'aot', type: Boolean },
-  { name: 'sourcemap', type: Boolean, aliases: ['sm'] },
+  { name: 'sourcemap', type: Boolean, aliases: ['sm', 'sourcemaps'] },
   { name: 'vendor-chunk', type: Boolean, default: true, aliases: ['vc'] },
   { name: 'base-href', type: String, aliases: ['bh'] },
   { name: 'deploy-url', type: String, aliases: ['d'] },