Add a toggle to deactivate the generation of the build() method to allow custom build()

[RustyNova016]

Currently, the builder autogenerate a build() method on all builders, which prevent having a custom build logic.
It would be nice to have a toggle to deactivate it, and letting us implement it ourself. Something like this:

#[derive(Builder)]
#[builder(no_build)]
struct MyType {
  integer: i32,
  string: String,
  optional: Option<String>,
}

impl MyTypeBuilder {
    pub fn build() -> Mytype {
        // Custom builder!
    }
}
```

[loganmzz]

While it might sounds legit, I'm not currently considering this feature. The main reason is that I need to specify type signature implementing the build() function for state-based builder.

The current version is close to stable but I haven't explore case where the built type has also type parameters. This will surely have impact on state-based type signature of implementation blocks.

I keep this in mind (and open :) ) for the time I will address parameterized built types.