Allow editor to set variables

[thibaultamartin]

Hebbot supports a template file with a jinja-like syntax. It supports a given list of variables corresponding to the actual news reports and various information.

Sometimes, the editors may want to add a placeholder in the template for something that is going to be updated every week (e.g. a link to a YouTube embed video).

It would be nice to have the following set of commands:

• !var set <key> "<value>" so {{<key>}} in the template is replaced by <value> when the editor issues !render
• !var unset <key> to unset the variable
• !var list to list all the variables set, and their corresponding value

I think it could also make sense to unset all the variables when the editor issues a !clear command