fix `versions` in case current version doesn't exist

./configure doesn't exist by default in repo: use autoconf to create it.

Gets rid of `doc/README.mdtoc` and its build script. Since GitHub.com
renders anchors for each heading, all we have to do is put a simple
table of contents into `README.md` itself, and everything will get
linked up nicely.

Pros of this approach:
* We don't have to point out to people not to edit `README.md` anymore
* We don't have to run the build script each time README gets edited

Cons of this change:
* The "chapter" numbers are lost. They were silly anyway.

`doc/mdtoc` renders a Markdown table of contents for a Markdown file.
`doc/filter-toc` filters that down to only headings after ToC.
This script can be used to easily insert ToC into the current document
when editing `README.md` with, e.g., Vim:

    :read !doc/filter-toc %

fix install instructions for ubuntu & other linux

fix obsolete usage instruction

`rbenv-versions` tries to read the current version to display a marker
next to it, but if that fails the whole script aborts.

This change makes it so that the failures from `rbenv-version-name` are
tolerated. It also makes the `--bare` mode never call it in the first
place, because it doesn't need to display a marker.

Fixed broken link to RVM in documentation

Added in short note on Uninstalling Rubies under new 2.4 section in README

All the changes to documentation should be done in doc/README.mdtoc

Just trying to keep the README.md up to date with Ruby versions

Installation Step 1 is a one liner

Changing Ruby versions references to latest versions to keep somewhat up-to-date.

Not a big change, just thought it's always good to make it easier for a beginner

remove "+=" operator in rbenv-rehash to support bash-3.0

help for versions and commands, including their options

Fix typos in documentation

Prevent $command from leaking outside of function

Add /usr/lib/rbenv/hooks to hook search path

This will help with the packaging of rbenv plugin that contain hooks in
Debian.

rbenv-init arguments should be allowed in any order

Conflicts:
	libexec/rbenv-rehash

Made ksh portability changes

Only run compctl if it's an interactive shell

Added specific message for ksh in identifying the proper shell
initialization file.

Changed rbenv functiond definition to be more portable.
Shell functions should be defined by using the function command or
using the parenthesis grammar, but using both is not portable:

rbenv() {...  -or-
function rbenv { ...

Based on the approach by @imajes

Quoting the argument to greadlink to make it work with paths containing spaces