urlselect: empty status when deactivated
pastebuf: allow opening unrecognized urls
urlselect: add scan mode parameter for `activate`

- `/urlselect activate` now accept an optional mode parameter to specify
  what buffer(s) should be scanned (overriding the *.scan_merged_buffers
  option). use `current` to scan only current buffer or `merged` to scan
  all buffers merged with the current one.

- readme: fix the keys of default custom commands
- readme: mention about buffer_* variable in hsignal
- readme: add note on how to override *.scan_merged_buffers option