Cleanup: code folds, variables; closes nima/site#21

Cleaned up the code presentation and folds, and changed all global
variables to force-check themselves; for example `${GLOBAL_VAR?}'.
Overhauled `module/xplm': pyenv, rbenv, plenv

The Python, Ruby, and Perl module managers are complete

This means that `rbenv', or `python-virtualenv' are no longer
os-level package dependencies; instead the `xplm' module takes
responsibility of installing anything to do with Ruby, Perl, or
Python via their respective `*env' virtual environment utilities,
including the installation of those utilities themselves.
Merge branch 'feature/xplm' into develop