Render templates with timeout: request for comments

[vgrigoriu]

We're going to use Pebble with untrusted templates. Among other measures we'd like to add a timeout on how long rendering a template can take, and stop it if it goes over. Current plan is to use an Executor Service.

@ebussieres, do you think this is something that might be useful for other Pebble users? Would it make sense to open a PR here or should we keep it to ourselves?