urlselect: support merged buffers

- there's a new option, scan_merged_buffers, if it's set to 1 script
  will also scan URLs from buffer that are merged with the current one

- added 2 bar items buffer_name and buffer_number

- check if register is successful and also check for weechat's version
  number

- changed the default custom commands from 1 and 2 to o and i