[Chrome Extension] Update content script to parse articles

[sungho-cho]

Add following functionalities to content script

• Detect articles and decide if it is possible to parse current page
  • Will be used to communicate to the popup script about failures
  • Users will see message like "Not enough text" or "Unable to generate summary"
• Extract content title and content body if applicable
• Pass parsed texts to background script upon success, so that the background script can pass them to backend API