Add dt (delayed tee), like tee but can output to the file that's being read from (for example sort foo.txt | dt foo.txt).

- Get just HELM_HOME from .bashrc.

- No more prerun files, remove said code.
- Set PS1 during __prompt (called by BASH_COMMAND everytime the prompt
is going to be shown).
- Consistent calculation of runduration across __prompt and
__command_notifier.
- Open bug: the exit status is shown until a new command is run.

This reverts commit 208c1bcc. Now all
commands are monitored.

- Use notify-send to notify on finished (successfully or not) long
running commands (all commands, not just those with monitor).
- Move all Bash specifics together, under the if shell==bash clause and
to the end.