Compile to build/ rather than output/

There are two issues with the `output/` directory:

* It has a nonstandard name: no other similar system uses a directory named `output/`.
* It has been used in older versions of nanoc as a place to store files that were not part of the compilation process. Such usage has been deprecated meanwhile, and `prune` removes files that are not part of the nanoc compilation process.

Two alternatives:

* `public/` is often used as the a directory containing files to be served through HTTP. Drawbacks of this name: 1) in some systems it is allowed and sometimes even encouraged to put files in there manually, which we want to prevent with nanoc; the name `public/` gives the impression that it is usable in the deprecated way; 2) it is used in the context of a web server, which nanoc is not.

* `build/` is a directory often used by build tools as the directory to write generated files too. The name makes it clear that no files are meant to be put in there that are not part of the compilation process.

After a brief discussion, I opted for changing `output/` to `build/`.

Relatedly, I believe `nanoc compile` should change to `nanoc build` for consistency.

CC @bobthecow