Add pyenv formula

// This pullreq is a re-submission of #17229 by @OiNutter.

Please add a pyenv formula. pyenv is a tool to manage multiple versions of python such like rbenv and ruby-build.

pyenv consists from following 2 formulas. They are copied from existing formula of rbenv and ruby-build, with little modifications for pyenv.

* pyenv
* python-build
 * The equivalent with ruby-build.  In contrast with ruby-build, python-build is bundled with pyenv as its default plugin.

This formula will create files in following paths.

* `#{HOMEBREW_PREFIX}/var/lib/pyenv/plugins`
 * Installed plugins for pyenv will be here. Same path structure with existing rbenv formula.
* `#{HOMEBREW_PREFIX}/var/lib/pyenv/versions`
 * Built python binaries will be here. Same path structure with existing ruby-build formula.