Add `change-extension` script

Add ability to change file extensions of files recursively in current
directory.
Set $VISUAL

Most tools look for $VISUAL before $EDITOR, and we tend to want that
anyway. This allows people to customize the two, for example using GUI
vim for $VISUAL and normal vim for $EDITOR, or more extreme: `ex -v` for
$VISUAL and `ex` for $EDITOR.
Revert to default Git log style

* Our current style hides message details
* Default style shows details and one-line versions as appropriate
Start tmux window numbers at 1

The keyboard from left to right is 1 2 3 4 5 6 7 8 9 0. Make tmux
windows match this order.
Update git's push.default from tracking to upstream

The 'tracking' value has been deprecated in favor of 'upstream'.
Renumber tmux windows after closing any of them
Add e alias for $EDITOR, v for $VISUAL

* Goal is to quickly open current project in vim to v.
* This is a dozens-of-times-per-day kind of task for me.

Lively discussion here:

https://github.com/thoughtbot/dotfiles/pull/100
Add `replace` script

Find and replace by a given list of files.
Use ag instead of ack

https://github.com/ggreer/the_silver_searcher

* ag is faster than ack
* ag searches all files by default (but still ignores gitignored files). This
  removes the need for ack's `--type-add=` options.
* The command is 33% shorter than ack!
g alias for git

With arguments, g acts like git.
Without arguments, it runs `git status`.

* Source functions/* after loading ZSH completion so `compdef` is available to g
  alias
Explain what's in dotfiles

* Link to fork/upstream process.
* Remove RVM reference.
* Don't create ~/.README.md in `install.sh` script.
Fix contributors link in README
Rename current file in vim buffer

* Retain relative path.
* Save file.
* Do not keep old file open in buffer.

Usage:

    :e old_file_name.rb
    :Rename new_file_name.rb
Change the 'git commit --amend' alias to 'git ca'