Spell-check Markdown by type, not file extension

Previously, only `*.md` files would get spell-checking. This change adds
`*.markdown` and decouples Markdown spell-checking by file extension.

http://robots.thoughtbot.com/vim-spell-checking/
Disable Rubygem documentation to make Bundler fast
Update Giant Robots blog post URLs in README
Only try to install vundle if it's not installed

Prevents error on re-running ./install.sh:

    fatal: destination path '~/.vim/bundle/vundle' already exists and is
    not an empty directory.

https://github.com/thoughtbot/dotfiles/issues/195
skip linking LICENSE file during install
Use rcm instead of ./install.sh

The rcm suite is designed to replace the `./install.sh` script found in
this and many other dotfiles repos across GitHub. By using rcm users can
combine multiple dotfiles repos, tag dotfiles, have host-specific
dotfiles, and other powerful features.

This commit removes `install.sh`, updates the README, adds a post-up
hook that vundles the vim bundles, and adds a `rcrc` configuration that
ignores `README.md` and `LICENSE` and sets the dotfiles directory to
just `dotfiles`.
Always update from vundle

Previously, the after-up hook would only do a `:BundleInstall` when
installing vundle for the first time. This is inconsistent with the
pre-rcm process, and also less convenient.
Cleanup zshrc

* remove duplicates with zlogin
* order by functions first, then bindings, then exports
* lowercase `setopt` arguments for consistency with `setopt` output
Merge changes from thoughtbot/dotfiles

Conflicts:

* install.sh
* zshrc