Allow bash-preexec to be included at any point

[rcaloras]

• Relaxes requirement that bash-prexec.sh needs to be included as last
  thing in bash_profile.sh
• Preserves existing prompt command in PROMPT_COMMAND variable. May make
  sense to still move this to a function in precmd.
• Should address Could the restriction that bash preexec is the last thing loaded be relaxed? #97 and Preserve existing DEBUG traps  #39
• Keeps existing traps preserved as preexec functions
• Removes trailing \n on PROMPT_COMMAND added for Incompatible prompt_command settings #58.

[rcaloras]

Doing some manual testing and added some unit tests and believe this should be good to go. cc @dimo414, @brandonweeks, @jombooth as relevant reviewers if anyone is free to take a look.

One potentially breaking aspect of this is:

• Removes trailing \n on PROMPT_COMMAND added for Incompatible prompt_command settings #58

Upon further inspection, I think appending a terminating character (e.g. ;, or \n) to the end of PROMPT_COMMAND is debatable. We actually chose against it in #41, and surveying a few other projects manipulating prompt command, it looks the standard is more towards adding terminating characters to the root when appending (i.e. PROMPT_COMMAND=$PROMPT_COMMAND\n new_function_being_appended).

[dimo414]

Apologies for not taking a look earlier. The PR looks great!