Immediately send <Esc> through in tmux

The default is 500 ms, which would only be useful if we used tmux
bindings that included <Esc>.
Restore custom undodir
Unbind C-b
Better way to set undodir

From :help dir:

   For Unix and Win32, if a directory ends in two path separators "//"
   or "\\", the swap file name will be built from the complete path to
   the file with all path separators substituted to percent '%' signs.
   This will ensure file name uniqueness in the preserve directory.  On
   Win32, when a separating comma is following, you must use "//", since
   "\\" will include the comma in the file name.

So we guarantee unique filenames. We're also talking advantage of
multiple arguments, so we'll use a CWD .vim/tmp folder if it's
available, or fall back to a system tmp folder.
Set undodir to a more appropriate location

Also, make sure that appropriate location exists :)

Per a conversation with tpope, here:

   https://github.com/hashrocket/dotmatrix/commit/6d0886aa81b897b3b4e656b72615bcf79428e314

I'm setting the undodir to an opt-in location. So that that opt-in
location is there by default for dotmatrix installs, I've added a
routine to create that directory if it's not present when you
install/upgrade.
Extract vim refactoring to weefactor plugin
Auto-bind <leader>l to extract RSpec let
Merge remote-tracking branch 'upstream/master'