Read list of custom sites from github repository json-file.
Sites which are less likely to end up in the main list of this extension (or less familiar sites currently in BP could also be transfered to this list).
These sites are loaded into BP options (by default disabled) and the allow_cookies/remove_cookies lists (background.js).
No more need to edit readme.md, background.js and options.js for these specific sites (just add line to sites_custom.json).
