apply un-used custom timeout value and update readme file

[showuon]

I found there's an un-used option timeout in the option list. In our project, we have an expected long waiting time when testing (ex: load a large script file), and want to custom the timeout value when testing.

Here's what I did:

1. apply un-used custom timeout value in page.setDefaultTimeout(timeout) after setup
2. set the default option timeout value to 30 secs to sync with puppeteer's deafult timeout value.
   So, this change won't affect existing user's tests running
3. update readme file to add timeout option description
4. add back an accidentally removed(I believe) allowFileAccess option content in readme file (in this PR: added --allow-file-access-from-files tag issue 298 #306)

reference: https://github.com/puppeteer/puppeteer/blob/master/docs/api.md#pagesetdefaulttimeouttimeout
The setDefaultTimeout will change the default maximum time for the following methods and related shortcuts:
page.goBack([options])
page.goForward([options])
page.goto(url[, options])
page.reload([options])
page.setContent(html[, options])
page.waitFor(selectorOrFunctionOrTimeout[, options[, ...args]])
page.waitForFileChooser([options])
page.waitForFunction(pageFunction[, options[, ...args]])
page.waitForNavigation([options])
page.waitForRequest(urlOrPredicate[, options])
page.waitForResponse(urlOrPredicate[, options])
page.waitForSelector(selector[, options])
page.waitForXPath(xpath[, options])

Thank you.

[showuon]

@steveoh , could you please help review this PR? Thank you.

[showuon]

hi @vladikoff , could you please help review this PR? Thank you.

[vladikoff]

see #320