Add ability to set/unset prompt prefixes and suffixes

Prefixes and suffixes are listed in a green bracket on the prompt,
if present. Primarily for virtualenv names and a few other use cases.

* precmd() adjusted accordingly
* prompt_prefix_add adds a string into the prompt prefix
* prompt_prefix_remove removes a prefix string
* prompt_suffix_add adds a string into the prompt suffix
* prompt_suffix_remove removes a suffix string
Use Python as json fallback if jq isn't installed