Context

I need to be able to configure page source for attachments in AllureSelenide. I use appium for automation of native mobile applications. On getPageSource() request Appium returns XML tree by default (and it can also be configured to return JSON), not HTML like Selenium WebDriver. So when my XML is treated as HTML, attachment is not displayed in Allure. It's just blank (because of source type mismatch).
So my pull request fixes this issue. By default HTML source type is implied but client can configure it to their needs using configureSource() method.

Checklist

• Sign Allure CLA
• Provide unit tests