💎🐵 add support for mandrill pipe template in mandrill option instead of handlebars

[gabrielstuff]

For mandrill template, the default template language we use in altruist is handlebars.
So you are forced to use {{}} instead of the standard pipe template language *||*.
We should provide an option in mandrill action to setup template language to pipe. This means adding merge_language key only if a custom template language is provided.