Add test coverage report

.gitignore

@@ -1 +1,2 @@
 node_modules
+.nyc_output

package.json

@@ -10,7 +10,7 @@
     "bench": "node bench/index.js",
     "preview": "node preview/serve.js",
     "pretest": "npm run lint",
-    "test": "tap test/*.spec.js --reporter spec"
+    "test": "tap test/*.spec.js --reporter spec --100"
   },
   "devDependencies": {
     "benchmark": "^2.1.4",

test/calc-text-width.spec.js

@@ -1,13 +1,23 @@
 const tap = require('tap')
 const calcWidth = require('../lib/calc-text-width.js').Verdana11
 
-tap.test('calc width for "npm"', t => {
+tap.test('basic functions', t => {
   t.ok(typeof calcWidth === 'function', 'export calcWidth function')
-  t.ok(Number.isFinite(calcWidth('npm')), 'result is a number')
+  t.ok(Number.isFinite(calcWidth('')), 'result is a number')
+  t.ok(calcWidth('') === 0, 'return 0 for empty string')
+  t.end()
+})
+
+tap.test('calc width for "npm"', t => {
   t.is(calcWidth('npm'), 24.9, 'result is correct value')
   t.end()
 })
 
+tap.test('calc width for unicode', t => {
+  t.is(calcWidth('壹佰贰拾叁'), 55, 'result is correct value')
+  t.end()
+})
+
 tap.test('exception handling', t => {
   t.throws(() => calcWidth(0), TypeError, 'throw if feed with non-string input')
   t.end()