Make displaying python version glyph in virtualenv segment configurable

[w-biggs]

Motivation: On my machine, python --version is taking 664ms to execute; this is then getting tacked onto every single shell command due to __bobthefish_prompt_virtualfish calling python --version to decide which version glyph to display. I'm not sure if python --version is simply not supposed to take that long, but I find it useful to have the current virtualenv displayed regardless, so making this configurable seems like the best route.

$theme_display_virtualenv_python_version is checked twice because I figured it was best to preserve the current behavior of "if the python version doesn't match any case, don't display the virtualenv color and glyph at all".

[bobthecow]

So with this change it shows the glyph if you're in a virtualenv, but no version number?

For other similar things we change the binary yes/no to include an option like verbose. I'd be inclined to do that here, rather than introduce a new variable just to remove the version.