Lightweight generator customization

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Writing a new class for generator is a bit boring, and most often, we only want to override the 'run' method. It's possible to create special rule that takes a user-provide rule to serve as generator's run rule.

For example:

build-indirectly spirit : @spirit-builder ;
# The parameters of the run as the same as for generator.run
rule spirit-builder ( name project ? : sources * : property-set )
{
   ....
}

It's necessary, though, to provide a way to call 'run' method of any existing generator from user-provided rule.

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This is implemented, except for docs.

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Documented.