Generator for asciidoc representation of all ECS fields (#347)

This code generates both the index page for the fields section, and the detailed page for each fieldset.

Also in this PR:

- If `short` not set, copy `definition` to it
- If `short` contains \n, fail. Beginning of an ECS linter.
- Beef up the local file imports in tests.
  - autopep8 ignore E402, in order to leave local imports alone
- Create generators/ecs_helpers.py, to start reusing functionality beween generators

Makefile

@@ -65,7 +65,7 @@ fields_legacy:
 # Format code and files in the repo.
 .PHONY: fmt
 fmt: ve
-	$(FIND) -name '*.py' -exec build/ve/bin/autopep8 --in-place --max-line-length 120 {} \;
+	$(FIND) -name '*.py' -exec build/ve/bin/autopep8 --ignore E402 --in-place --max-line-length 120 {} \;
 	go get golang.org/x/tools/cmd/goimports
 	goimports -w -l -local github.com/elastic $(shell $(FIND) -name '*.go')