Update external completions
Update external history-substring-search
Update external syntax-highlighting
Replace pythonz with pyenv

Depending on Python to manage Python is silly.
Add utility aliases for Cygwin

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Remove duplicate alias definitions
Simplify a conditional
Merge branch 'master' of github.com:sorin-ionescu/prezto into git-module-aliases