Typo
Use `jk` to exit insert mode

In addition to defining this new mapping, this change also includes the
"retrain my brain" section. This section is intended to disable mappings
that I'm trying to break myself of the habit of using. It begins by
disabling `<esc>` in insert mode.
Add some things from Learn Vimscript the Hard Way

These changes may eventually make their way into the regular part of my
.vimrc file, but for now I'm keeping all of the LVTHW stuff in one
place.