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!
    }
}
```