Suggest keybinding configuration for Spacemacs Users

[rymndhng]

For spacemacs user, it's desirable to have keybindings that work with the major-mode prefix. The Configuration section of the README should include instructions on how to set this up.

I have this little script for doing this, however it'd be fantastic if this could be packaged up more nicely.

(progn
      (dolist (major-mode '(typescript-mode js2-mode))
        (spacemacs/set-leader-keys-for-major-mode major-mode
          "tp" 'jest-test-run-all-tests
          "ta" 'jest-test-rerun-test
          "tn" 'jest-test-run
          "tt" 'jest-test-run-at-point)
        (spacemacs/declare-prefix-for-mode major-mode "mt" "test")))