Add coverage testing

Author: wooorm

.gitignore

@@ -2,3 +2,5 @@
 node_modules
 tmp
 *.log*
+coverage
+.nyc_output

package.json

@@ -35,21 +35,31 @@
     "xtend": "^4.0.1"
   },
   "devDependencies": {
+    "hastscript": "^3.0.1",
     "is-hidden": "^1.1.0",
     "mdast-util-assert": "^1.0.0",
     "negate": "^1.0.0",
+    "nyc": "^10.1.2",
     "rehype-parse": "^2.0.1",
     "remark-parse": "^2.2.0",
     "remark-stringify": "^2.3.0",
     "tape": "^4.6.3",
     "unified": "^5.1.0",
+    "unist-builder": "^1.0.2",
     "unist-util-remove-position": "^1.1.0",
     "xo": "^0.17.1"
   },
   "scripts": {
     "lint": "xo",
     "test-api": "node tests",
-    "test": "npm run lint && npm run test-api"
+    "test-coverage": "nyc --reporter lcov tape tests/index.js",
+    "test": "npm run lint && npm run test-coverage"
+  },
+  "nyc": {
+    "check-coverage": true,
+    "lines": 100,
+    "functions": 100,
+    "branches": 100
   },
   "xo": {
     "space": true