Combine zlogin settings into zshrc

* easier when everything is just one file
* most (if not all) interactive shells are login shells and we likely want those
  settings for interactive shells
Remove leftover snippets variable
Also look for dotfiles-local

While trying out the approach outlined in Croaky's latest [blog post], I
realized that running `rcup` would not also pull updates from my local
extensions. If you clone your extensions as `dotfiles-local`, this
update will pick them up.

Non-existent directories are ignored by rcup, so it won't cause issues
if you don't have `dotfiles-local`.

[blog post]: http://robots.thoughtbot.com/manage-team-and-personal-dotfiles-together-with-rcm
Don't attempt to autocorrect spelling

This frequently catches false positives for aliases, particularly for
subcommands. It also takes just as long to look at the autocorrect
prompt and select "y" as it does to just retype the command yourself.
Use default 'complete' Vim setting

Default setting is `.,w,b,u,t,i`:

* `.`: scan the current buffer ('wrapscan' is ignored)
* `w`: scan buffers from other windows
* `b`: scan other loaded buffers that are in the buffer list
* `u`: scan the unloaded buffers that are in the buffer list
* `t`: tag completion
* `i`: scan current and included files

The default setting is more likely to find a useful match, and modern
machines can search many open and unloaded buffers without pausing.
Disable zsh correction of command arguments
Add local gitconfig
Don't use zsh vim mode
Add homebrew bin & sbin to PATH
Add some packages to Brewfile
Add git aliases for diff and diff --cached