Adds hook scope aliases `example` and `context`

Created by: fj

This allows for nicer, more sensible naming in the context of RSpec.configure blocks and elsewhere.

For example, this block sounds like it's a hook that runs once, before any example:

RSpec.configure do |config|
  config.before(:all) do       # ... confusing :(
    # do something
  end
end

But it's actually a hook that runs once before every context, which is confusing. So a better name would be:

RSpec.configure do |config|
  config.before(:context) do   # sensible!     :D
    # do something
  end
end

This change adds, in hook scopes, the ability to use :example as an alias for :each and :context as an alias for :all.

See #297 for further discussion.