Add RUBY_INSTALL_* env variables to allow configuration

[postmodern]

Based on the discussion in #413, maybe add RUBY_INSTALL_* env variables to allow global configuration of ruby-installs defaults in a user's shell configuration.

Suggestions

• RUBY_INSTALL_RUBIES_DIR - overrides the default directory to install rubies into (either /opt/rubies for root, or ~/.rubies for regular users).
• RUBY_INSTALL_SRC_DIR / RUBY_INSTALL_CACHE_DIR - overrides where ruby archives will be downloaded to (aka /usr/local/src for root, and ~/src/ for regular users). Eventually, I want to switch to downloading into ~/.cache/ruby-install/... for regular users to comply with XDG and get rid of the unsightly ~/src directory.
• RUBY_INSTALL_PACKAGE_MANAGER / RUBY_INSTALL_PKG_MANAGER - overrides the detected package manager. Would be useful when you have both homebrew and MacPorts installed, or apt and LinuxBrew installed.
• RUBY_INSTALL_DOWNLOADER - overrides whether to use wget or curl. Maybe some people are really opinionated about using curl or wget, but have both installed?

[postmodern]

Also relevant to #253, #284, and #177.

[postmodern]

Added RUBY_INSTALL_RUBIES_DIR and RUBY_INSTALL_SRC_DIR in 43e837b. Added RUBY_INSTALL_PKG_MANAGER in 74e1912. Decided against adding RUBY_INSTALL_DOWNLOADER, unless there is popular demand for it.