some fixes

Mutable class attribut leading each instance to share transformers

default value to False for ActivatableTransformers

Exception on non-existing attribute

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I still have some concern about another issue. 

There is no-way with current architecture to have different default for the **transformers** depending on which  subclass of `Exporter` you init. Which in the end will lead to have at least one of the subclass unusable, or another consuming ridiculous amount of memory for nothing depending on what you want to do.

As examples, you cannot have both RstExporter extract figures to resources folder, and BaseHtmlExporter not doing it.

Or you cannot, have LaTeX Prefer LaTex output and Markdown Png...

You see my point....