Adds pygments library for html formatting of the plain text result preview

Pygments is a python source code highlighter http://pygments.org/docs/ . The idea is to use highlighting tools when previewing the plain text search results. The changes added perform the guess on the source code language based on the previewed contents and if match found - highlight the preview as html page with formattng and line numbers. If there is no pygments installed in the system the code falls back to the plain text.