Don't overwrite `:wrap_runtime` when using #wtf? and allow multiple runtime extensions instead

[apotonick]

Currently, when using #wtf?, the :wrap_runtime option is overwritten:

trailblazer-developer/lib/trailblazer/developer/trace.rb

Line 28 in 404deb6

wrap_runtime: ::Hash.new(Trace.task_wrap_extensions), # DISCUSS: this overrides existing {:wrap_runtime}.

Instead of supporting a pure Hash for this option, we should have a mechanism similar to the DSL (wrap_static) where the Ext() options are automatically merged. The latter could happen in trace.rb.