push command output updating into background process

 - this rate limits update rate since updates are no longer triggered by browser
 - this also allows browser request to turn around faster since it doesn't cause
   user command rerun
 - this allows open multiple browser pages pointing to one endpoint
 - no need for CGI anymore, command output is just a text file
 - added DEBUG option, set DEBUG=1 env to use

In order to achieve smooth output updates and uploads two files created for
capturing output data.0 and data.1 . The file which is pointed by symlink data
is called active and served upon request. Another file called shadow is used to
capture command output. Once update is done symlink is switched atomically.