feat(Frame): introduce Frame.select

This patch adds `Frame.select` method that does the same functionality as
former `Page.select`, but on a per-frame level.

The `Page.select` method becomes a shortcut for the ÷main frame's select.

Fixes #1139

docs/api.md

@@ -128,6 +128,7 @@
   * [frame.isDetached()](#frameisdetached)
   * [frame.name()](#framename)
   * [frame.parentFrame()](#frameparentframe)
+  * [frame.select(selector, ...values)](#frameselectselector-values)
   * [frame.title()](#frametitle)
   * [frame.url()](#frameurl)
   * [frame.waitFor(selectorOrFunctionOrTimeout[, options[, ...args]])](#framewaitforselectororfunctionortimeout-options-args)
@@ -932,13 +933,15 @@ Shortcut for [page.mainFrame().executionContext().queryObjects(prototypeHandle)]
 - returns: <[Promise]<[Array]<[string]>>> Returns an array of option values that have been successfully selected.
 
 Triggers a `change` and `input` event once all the provided options have been selected.
-If there's no `<select>` element matching `selector`, the method throws an error. 
+If there's no `<select>` element matching `selector`, the method throws an error.
 
 ```js
 page.select('select#colors', 'blue'); // single selection
 page.select('select#colors', 'red', 'green', 'blue'); // multiple selections
 ```
 
+Shortcut for [page.mainFrame.select()](#frameselectselector-values)
+
 #### page.setContent(html, options)
 - `html` <[string]> HTML markup to assign to the page.
 - `options` <[Object]> Navigation parameters which might have the following properties:
@@ -1504,6 +1507,19 @@ If the name is empty, returns the id attribute instead.
 #### frame.parentFrame()
 - returns: <[Frame]> Returns parent frame, if any. Detached frames and main frames return `null`.
 
+#### frame.select(selector, ...values)
+- `selector` <[string]> A [selector] to query frame for
+- `...values` <...[string]> Values of options to select. If the `<select>` has the `multiple` attribute, all values are considered, otherwise only the first one is taken into account.
+- returns: <[Promise]<[string]>>
+
+Triggers a `change` and `input` event once all the provided options have been selected.
+If there's no `<select>` element matching `selector`, the method throws an error.
+
+```js
+frame.select('select#colors', 'blue'); // single selection
+frame.select('select#colors', 'red', 'green', 'blue'); // multiple selections
+```
+
 #### frame.title()
 - returns: <[Promise]<[string]>> Returns page's title.