[Enhancement] Enable selected transformers in formatted file

[gohierf]

It would be nice to be able to enable a transformer or another from within the file.

For instance, let's say my robotidy configuration files states that SmartSortKeywords transformer is disabled.

Then maybe for some files I actually want to activate it.

I am using VSCode format on save, rather than manually running robotidy now and then, or from the CI.

It would be convenient that I can activate a transformer using a comment, like it is possible for robocop rules.
# robocop: on=rule1

For our example here, something like:

# robotidy: enable=SmartSortKeywords (on is as good as enable as far as I can tell).

Which could be added at the beging of the file, or in the *** Keywords *** section.