This is a new variable that will need to be set on all new prompts and
is not backwards compatible with custom prompts that are not prezto
managed, but use prezto's editor-info functionality. Updated the
README.md with additional information for themes.
Changes:
* In prezto, function names are hyphenated ('-') by convention,
rename `promptpwd` to `prompt-pwd` accordingly.
* Unset local variable `current_pwd` proactively.
* Tweak additional documentation for `prompt-pwd`.
