Adds an optional peer dependency

PR-URL: #34
Credit: @arcanis
Close: #34
Reviewed-by: @isaacs

Author: arcanis

fixtures/npm-test-peer-deps/0.0.0.json

@@ -1 +1 @@
-{"author":{"name":"Domenic Denicola"},"name":"npm-test-peer-deps","version":"0.0.0","peerDependencies":{"request":"0.9.x"},"dependencies":{"underscore":"1.3.1"},"_id":"[email protected]","dist":{"shasum":"82f3ccba11914dc88bcb185ee3b1b33b564272bc","tarball":"http://registry.npmjs.org/npm-test-peer-deps/-/npm-test-peer-deps-0.0.0.tgz"},"_from":".","_npmVersion":"1.3.25","_npmUser":{"name":"domenic","email":"[email protected]"},"maintainers":[{"name":"domenic","email":"[email protected]"}],"directories":{}}
+{"author":{"name":"Domenic Denicola"},"name":"npm-test-peer-deps","version":"0.0.0","peerDependencies":{"request":"0.9.x","left-pad":"*"},"peerDependenciesMeta":{"left-pad":{"optional":true}},"dependencies":{"underscore":"1.3.1"},"_id":"[email protected]","dist":{"shasum":"82f3ccba11914dc88bcb185ee3b1b33b564272bc","tarball":"http://registry.npmjs.org/npm-test-peer-deps/-/npm-test-peer-deps-0.0.0.tgz"},"_from":".","_npmVersion":"1.3.25","_npmUser":{"name":"domenic","email":"[email protected]"},"maintainers":[{"name":"domenic","email":"[email protected]"}],"directories":{}}

fixtures/npm-test-peer-deps/latest.json

@@ -1 +1 @@
-{"author":{"name":"Domenic Denicola"},"name":"npm-test-peer-deps","version":"0.0.0","peerDependencies":{"request":"0.9.x"},"dependencies":{"underscore":"1.3.1"},"_id":"[email protected]","dist":{"shasum":"82f3ccba11914dc88bcb185ee3b1b33b564272bc","tarball":"http://registry.npmjs.org/npm-test-peer-deps/-/npm-test-peer-deps-0.0.0.tgz"},"_from":".","_npmVersion":"1.3.25","_npmUser":{"name":"domenic","email":"[email protected]"},"maintainers":[{"name":"domenic","email":"[email protected]"}],"directories":{}}
+{"author":{"name":"Domenic Denicola"},"name":"npm-test-peer-deps","version":"0.0.0","peerDependencies":{"request":"0.9.x","left-pad":"*"},"peerDependenciesMeta":{"left-pad":{"optional":true}},"dependencies":{"underscore":"1.3.1"},"_id":"[email protected]","dist":{"shasum":"82f3ccba11914dc88bcb185ee3b1b33b564272bc","tarball":"http://registry.npmjs.org/npm-test-peer-deps/-/npm-test-peer-deps-0.0.0.tgz"},"_from":".","_npmVersion":"1.3.25","_npmUser":{"name":"domenic","email":"[email protected]"},"maintainers":[{"name":"domenic","email":"[email protected]"}],"directories":{}}