add POWERLEVEL9K_VIRTUALENV_SHOW_VERSION option to virtualenv segment

[iilonmasc]

As discussed in #808
This PR adds a new option to the virtualenv segment.

If you add POWERLEVEL9K_VIRTUALENV_SHOW_VERSION=true to your .zshrc p9k will add your python version to your venv segment

It looks like this:
Using Python2

Using Python3

Thanks @hiimdoublej for suggesting this!

[dritter]

Thanks @sambadevi for this PR!

To be honest, I am a bit ambivalent with this PR. On one hand 👍 for creativity (I personally never thought about rendering another segment within one).
If it would be possible to set a condition (which there is, if #771 will be merged), and if there would be a python_version segment that prints out the version, we would get the exact same result (if the colors match and the icon would be set to empty, of course), and it would be combinable with other python env segments..

[iilonmasc]

So @dritter your suggestion is creating a new segment python_version and wait for #771 to add conditionals? I can change this PR to be a separate segment, the code wouldn’t differ that much. But there won’t be any segment combination unless #771 gets merged.

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I'd like to stay with this functionality tbh, my intention wasn't to add a new segment just for the python version but to show the python version only if I am in a virtualenv (because I am using both, python2 and 3 in different virtualenvs).
Adding a new segment for pyver would probably help, however adding the virtualenv-only logic in there would need a new variable to control the pyver segment. It's more like a workaround in that case

[dritter]

For the records: The successor of #771 is merged and it is possible to define own conditions for every segment. The condition should look like this P9K_PYTHON_VERSION_CONDITION='[[ -n "${VIRTUAL_ENV}" ]]'. Yes, this is basically a duplication of the condition in the virtualenv segment; that is the trade-off..
That a separate python_version segment makes sense is shown in #1053 .

[bhilburn]

Per @dritter, the project's structure is completely changed in the next branch. It should be pretty easy to port this change over, though!

[iilonmasc]

I am closing this PR and will port this change to the new structure in next