Use docile to simplify dsl setup

[timolehto]

Using Docile https://github.com/ms-ati/docile instead of cooking our own DSL building logic would have some benefits:

• Maybe more standard DSL as other libraries also use it.. e.g. at lest pena already has docile as some of its gems dependencies.
• A few less lines for piperator to maintain as Builder now significantly simplified
• In addition to current things we can do with DSL would also allow referencing instance variables, albeit setting the variables from nested blocks is not possible, so in piperators case e.g. a @counter can not really be used

On the other hand it's an extra dependency to a small utility library which so far has had none. I can certainly see the appeal in keeping things that way. Also the instance variable handling has raised some concerns https://github.com/ms-ati/docile/issues/37