The `python` Homebrew package installs unversioned links to Python 3
executables in `/usr/local/opt/python/libexec/bin`. Add this directory
to the `PATH` environment variable when it's present.
Tips from [zsh-vimto](https://github.com/laurenkt/zsh-vimto).
* Remove delay when changing mode
* Enable backspace when returning to viins from vicmd mode
After fixing the macOS `/etc/zprofile` issue rewriting the `PATH`
environment variable it is no longer necessary to explicitly add
`/usr/local/opt/python/libexec/bin` to `PATH` since Homebrew installed
python 2.x is installed in `/usr/local/bin` and is used by default.
macOS is obnoxious and overwrites the PATH from a users ~/.zshenv, which is
sourced first, in /etc/zprofile by calling `/usr/libexec/path_helper -s` so
this is required so that PATH is once again set to the desired value.
Add ccache symlink directory to start of PATH, this must be after
`typeset -U PATH` because on macOS this reorders the list so ccache's
symlinks will no longer be at the start of PATH rendering it unusable.
