Add support proxy

src/client.js

@@ -16,6 +16,8 @@ function ClearbitClient (config) {
   this.key = config.key || process.env.CLEARBIT_KEY;
   assert(!!this.key, 'An API key must be provided');
 
+  this.proxy = config.proxy || null;
+
   this.Company = require('./enrichment/company').Company(this);
   this.Person = require('./enrichment/person').Person(this);
   this.Enrichment = require('./enrichment').Enrichment(this);
@@ -71,7 +73,8 @@ ClearbitClient.prototype.request = function (options) {
       timeout: timeout,
       username: this.key,
       password: '',
-      user_agent: 'ClearbitNode/v' + pkg.version
+      user_agent: 'ClearbitNode/v' + pkg.version,
+      proxy: this.proxy
     }
   )
   .bind(this)