helm-projectile.el: rewrite `helm-projectile-define-key' to allow defini...

Hi Bozhidar,

I just rewrote the macro `helm-projectile-define-key` that I had written previously, to allow defining multiple keys. I then used the enhanced macro to clean up the code where `helm-projectile-define-key` are being called repeatedly while defining multiple key bindings.

To illustrate, when previously we have to write:

    (helm-projectile-define-key KEYMAP KEY1 DEF1)
    (helm-projectile-define-key KEYMAP KEY2 DEF2)
    (helm-projectile-define-key KEYMAP KEY3 DEF3)

Now, we can clean it up to:

    (helm-projectile-define-key KEYMAP
       KEY1 DEF1
       KEY2 DEF2
       KEY3 DEF3)

Could you please merge it if you agree with my improvement?


York Zhao