Fix speed issues for bash subshells
Fixes issue #5 https://github.com/jml/undistract-me/issues/5

The issues seems to be that pre-exec thinks we are running in a subshell in
this case.  Remove this code to always clear the debug trap irrespective of
bash subshell level.

Before we have this:

  % time (for i in {0..512}; do true; done)

  real    0m8.657s
  user    0m0.424s
  sys     0m0.976s

After we get:

  % time (for i in {0..512}; do true; done)

  real    0m0.026s
  user    0m0.012s
  sys     0m0.000s
New function to log the exit code.
Log the exit code of a command that is run
Use exit code of command exited to change notification.
export __preexec_exit_status status so custom commands can use it also
remove the effect of HISTTIMEFORMAT on the command name
Fix setting exit code when there is no user defined PROMPT_COMMAND