Add environment variable completion to counsel-find-file

[CeleritasCelery]

Closes #776. In that issue @Henry created some code to add environment variable completion to counsel-find-file. @abo-abo asked if he or @TauPan could make a PR. Since it has been several months, I took the opportunity to make a PR. I also simplified the code and made it more performant.

[CeleritasCelery]

I moved back to the original implementation after applying feedback. If @abo-abo or someone knows how to make the quitting recursive minibuffers work in the post-command-hook we can move to the ivy--exhibit solution instead. The code is at CeleritasCelery/swiper@956fcfd

[CeleritasCelery]

I realized that this implementation does not work with read-file-name-internal. While we could add this to the that file map, I feel like the magic solution would work best if we could get it working correctly.

[abo-abo]

@CeleritasCelery Thanks, please test.

I've re-written a portion of your code to simplify it. I think it now falls into the 15 lines limit that's exempt from an Emacs CA. But you'll need a CA for future (hopefully) contributions.

[CeleritasCelery]

Thanks @abo-abo. I have already applied for a CA. Hear it takes about a month.

I am impressed how much you were able to simplify what I had added.

I only see two issues:

1. if navigating to a directory via env variable, it will only navigate to parent directory and select the variable. Most environment variables don't have a trailing slash. That is why I added this code

(when (file-accessible-directory-p path)
    (setq path (file-name-as-directory path))

2. This environment variable completion does not work in the read-file-name handler. I looked into how to make this work but was not able to figure it out. I added this to the magic handlers in ivy--exhibit but then you can't abort the env read.

[abo-abo]

@CeleritasCelery I've made a change to ivy.el with a recent commit, after that e.g. read-file-name works if I bind $ in ivy-minibuffer-map.

Please add the path expansion check as a new PR.