Implement procedure grouping

[aosasona]

We need to introduce a new feature to perform certain operations more conveniently on only a subset of procedures, for example, adding middleware functions or excluding middleware functions.

This will probably be defined as:

type ProcedureGroup []Procedure

func (r *Robin) Group(procedures ...Procedure) ProcedureGroup

A few methods like WithMiddleware and ExcludeMiddleware at least will be made available on the ProcedureGroup function.