maint/CI ~ add code coverage support and reporting

- `nyc` for code coverage testing/reporting
- CodeCov.io used for coverage report aggregation
  - note: Coveralls.io can't merge multiple CI builds (needed for multi-platform coverage support)
  - ref: <lemurheavy/coveralls-public#613>

.gitignore

@@ -1,2 +1,3 @@
+.nyc_output
 node_modules
 yarn.lock

package.json

@@ -12,7 +12,8 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava && tsd"
+		"coverage": "nyc report --reporter=text-lcov | codecov --disable=gcov --pipe",
+		"test": "xo && nyc --silent ava && tsd"
 	},
 	"files": [
 		"index.js",
@@ -40,7 +41,9 @@
 	],
 	"devDependencies": {
 		"ava": "^1.4.1",
+		"codecov": "^3.5.0",
 		"eslint": "^5.16.0",
+		"nyc": "^14.1.1",
 		"tsd": "^0.7.1",
 		"xo": "^0.24.0"
 	}