feat(Input): Add keyboard methods to elementHandle (#801)

This patch:
- adds input methods to ElementHandle, such as ElementHandle.type and ElementHandle.press
- changes `page.type` to accept selector as the first argument
- removes `page.press` method. The `page.press` is rarely used and doesn't operate with selectors; if there's a need to press a button, `page.keyboard.press` should be used.

BREAKING CHANGE: `page.type` is changed, `page.press` is removed.

Fixes #241.

lib/Input.js

@@ -88,6 +88,32 @@ class Keyboard {
       unmodifiedText: char
     });
   }
+
+  /**
+   * @param {string} text
+   * @param {{delay: (number|undefined)}=} options
+   */
+  async type(text, options) {
+    let delay = 0;
+    if (options && options.delay)
+      delay = options.delay;
+    for (const char of text) {
+      await this.press(char, {text: char, delay});
+      if (delay)
+        await new Promise(f => setTimeout(f, delay));
+    }
+  }
+
+  /**
+   * @param {string} key
+   * @param {!Object=} options
+   */
+  async press(key, options) {
+    await this.down(key, options);
+    if (options && options.delay)
+      await new Promise(f => setTimeout(f, options.delay));
+    await this.up(key);
+  }
 }
 
 class Mouse {