Automatic local exec

Fixes #119

Fixes #294

This is to insure portability to ksh.

Fixes #205, references #209

Should be `/usr` or `/usr/local`, not `/usr/bin` or `/usr/local/bin`

Fixes #274

Closes #263

Don't have `command -v` abort the whole script prematurely.

Use `rbenv-commands` instead of `rbenv commands`. The latter
unnecessarily goes through `rbenv` executable.

Fixes #254

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