.gitconfig: Make `git rebase` safer on OS X

See <http://www.git-tower.com/blog/make-git-rebase-safe-on-osx/>.

Closes #291.
.aliases: Improve `lsd`

Ref. #292.
README: Use `Brewfile` instead of `.brew`

Closes #294.
.osx: Hide Safari’s sidebar in Top Sites by default

Closes #296.