Add preexec.bash to get ZSH-like 'preexec' and 'precmd' functions

Taken from http://www.twistedmatrix.com/users/glyph/preexec.bash.txt.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
preexec.bash: Fix the case when $PROMPT_COMMAND is empty

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
preexec.bash: Trim leading / trailing whitespaces from the command

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Add a preexec script to directly execute batch files

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>