bash: don't touch the promptvars option
bash: don't touch the promptvars option

Rationale:

* In general, it is not proper for liquidprompt to change shell
  options that it does not absolutely have to change.

* LP_PS1_{PREFIX,POSTFIX} are a thing, and are in fact an effective
  means to combine liquidprompt with other prompt-touching logics
  (such as emacs-vterm).  So it is not good to prevent those other
  logics from actually working.

This commit restores the bash version of __lp_escape to what it did
prior to 62f0270 (escaping dollar signs in addition to backslashes)
and removes all interference with the promptvars shell option.

NOTE: I don't know much about shells other than bash, so left the
parallel logic for zsh as is.

laurentbardin/liquidprompt,milouse/liquidprompt,nojhan/liquidprompt,jaesivsm/liquidprompt