[O] Let ob-shell support specify shell to execute shell code?

[stardiviner]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

I want to specify Shell for ob-shell.

Like this src block:

#+begin_src shell
echo $SHELL
#+end_src

I found ob-shell has header argument `:shebang`. But it only is used for 
tangled script.

I hope ob-shell can support specify shell in header argument like this:

#+begin_src shell :shell "bash"
echo $SHELL
#+end_src

Because some code is specified to Bash, or Zsh or other Shells.

And I found ob-shell.el has variable `org-babel-shell-names`. But seems
it is not used at my case.
-----BEGIN PGP SIGNATURE-----

iQEzBAEBCAAdFiEE8J9lDX1nSBmJJZFAG13xyVromsMFAlrAdgsACgkQG13xyVro
msNeowgApFNh2uVJDjym+MZTk5oWy6K2dJfquBNXiGBNISarhfl4w+JRVHebubu+
BRmwpwgsa2YNF7sVOO7bbJPNEIykOOvmyBQDVM7RA2VWKWxvmmhBerisMsYWD4OD
6OBSoVKA+DGqQmrCd05YHiO2vLcIfuy+34lTAQsbK/+iHH1bfZmER5n/BvywVaqg
Ba44Z2343kfgQ6n/h19K1HKF6bL/FpQZ7L4NWIJGQu81WjxXIpEoT/iPX/duisii
75GJWsVhTA3IldNc+1nakHYYMYN/O5fkEavXsnCmtQEJlk2le0MIxbMHgEz3/vjG
0NxYLRKMllz/A+3so4/M7CV9PLXl7w==
=alpp
-----END PGP SIGNATURE-----