bashrc: use PROMPT_COMMAND env var for readability

refs:
- https://wiki.archlinux.org/index.php/Color_Bash_Prompt
- http://stackoverflow.com/questions/16715103/bash-prompt-with-last-exit-code
bashrc: output last command's exit status in PS1

refs:
- https://wiki.archlinux.org/index.php/Color_Bash_Prompt
- http://stackoverflow.com/questions/16715103/bash-prompt-with-last-exit-code
- http://stackoverflow.com/questions/22361722/simplifying-advanced-bash-prompt-variable-ps1-code/22362727#22362727
bashrc: DRY by introducing `source-if` function