Add support for customizing jet-error-buffer-name

[danielhvs]

In my workflow I'd like to control jet-error-buffer-name var so that I can just see the error in stdout.
I propose to either:

1. Add an example like the below in the README.md to show off this possibility

(use-package jet
  :custom
  (jet-error-buffer-name nil))

2. Or make it customizable, like so:

(defcustom jet-error-buffer-name "*jet error*"
  "When `nil`, redirects to stdout"
  :group 'jet
  :type 'string)

[ericdallo]

Added the defcustom here

[danielhvs]

Thanks, @ericdallo!