xclipurl: shorten prompt
xclipurl: rename to urlselect+support tmux buffer
urlselect: fix filename
urlselect: cancel selection when switching buffer

- add `buffer_switch` hook to cancel the url selection when switching to
  another buffer.

- use a global active_buffer variable to specify the buffer for url
  selection. this also removes the buffer arg in some functions.

- add script unload function

- sed s/xclipurl/urlselect/ README.md