Podman support

[ceblan]

Hi @KeyWeeUsr,

first of all, thanks for this package.

Just in case you wish to merge it, I've made a small change in order to made it compatible with Podman.

It simply adds this custom variable (which defaults to "docker"):

(defcustom mermaid-docker-engine
  "docker"
  "Container engine."
  :group 'mermaid-docker
	:type '(string :options ("docker" "podman"))
	:set 'set-default)

and uses it instead of the hardcoded "docker" literal.

I,ve also noted how to configure it in the README.md.

Regards.