test: exclude write-coverage from coverage report

Added a .nyrc configuration file that can be used to configure
test coverage.

Added an exclude rule that removes write-coverage.js from coverage
reports.

Pulled reporter configuration into .nycrc and added an additional
text reporter.

.gitignore

@@ -12,6 +12,7 @@
 !.gitignore
 !.gitkeep
 !.mailmap
+!.nycrc
 !.remarkrc
 
 core

.nycrc

@@ -0,0 +1,6 @@
+{
+  "exclude": [
+    "**/internal/process/write-coverage.js"
+  ],
+  "reporter": ["html", "text"]
+}

Makefile

@@ -165,7 +165,7 @@ coverage-test: coverage-build
 	$(NODE) ./node_modules/.bin/istanbul-merge --out \
 		.cov_tmp/libcov.json 'out/Release/.coverage/coverage-*.json'
 	(cd lib && .$(NODE) ../node_modules/.bin/nyc report \
-		--temp-directory "$(CURDIR)/.cov_tmp" -r html \
+		--temp-directory "$(CURDIR)/.cov_tmp" \
 		--report-dir "../coverage")
 	-(cd out && "../gcovr/scripts/gcovr" --gcov-exclude='.*deps' \
 		--gcov-exclude='.*usr' -v -r Release/obj.target/node \