Option to add boolean attributes

[shmargum]

I would like to add some boolean attributes. They are not standard, but i would like to specify them as an option in the config.

For example, to render a valid version of my page on Google AMP, I need to include several custom boolean attributes, and the page does not validate if they are not exactly as described. These attributes include amp ⚡ and other amp specific attributes.

[k0kubun]

For some performance reasons, I don't want to add an option for that. Haml is slow because the number of options required on runtime is large. The option must make performance worse unless the option is a singleton.

In addition, I don't want to add method calls on runtime. So we should modify the single constant directly. This is the plugin for you. https://github.com/hamlit/hamlit-boolean_attributes