Add option to force convert linked page to unicode

There is error when viewing linked pages on non-unicode site. For example: 4pda.ru, that have windows-1251 charset. 
This patch allows to convert fetched page to unicode for proper view.

Example of usage:

    "4pda.ru": {
         "type": "xpath",
         "xpath": "div[@class='post']/div[@class='entry']",
         "force_charset": "windows-1251",
         "force_unicode": true
    }