implemented an accessser of #asEmailString() (#158)

lib/accessors/email-string.js

@@ -0,0 +1,16 @@
+'use strict'
+
+const asString = require('./string')
+
+// eslint-disable-next-line no-control-regex
+const EMAIL_REGEX = /^(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\u0001-\u0008\u000b\u000c\u000e-\u001f\u0021\u0023-\u005b\u005d-\u007f]|\\[\u0001-\u0009\u000b\u000c\u000e-\u007f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9]))\.){3}(?:(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])|[a-z0-9-]*[a-z0-9]:(?:[\u0001-\u0008\u000b\u000c\u000e-\u001f\u0021-\u005a\u0053-\u007f]|\\[\u0001-\u0009\u000b\u000c\u000e-\u007f])+)\])$/
+
+module.exports = function asEmailString (value) {
+  const strValue = asString(value)
+
+  if (!EMAIL_REGEX.test(strValue)) {
+    throw new Error('should be a valid email address')
+  }
+
+  return strValue
+}

lib/accessors/index.js

@@ -24,5 +24,7 @@ module.exports = {
   asString: require('./string'),
 
   asUrlObject: require('./url-object'),
-  asUrlString: require('./url-string')
+  asUrlString: require('./url-string'),
+
+  asEmailString: require('./email-string')
 }

test/index.js

@@ -24,6 +24,7 @@ describe('env-var', function () {
     ARRAY_WITH_DELIMITER_PREFIX: ',value',
     DASH_ARRAY: '1-2-3',
     URL: 'http://google.com/',
+    EMAIL: 'email@example.com',
     ENUM: 'VALID',
     EMPTY_STRING: '',
     EMPTY_STRING_WITH_WHITESPACE: '  '
@@ -584,6 +585,21 @@ describe('env-var', function () {
     })
   })
 
+  describe('#asEmailString', function () {
+    it('should return an email address', function () {
+      expect(mod.get('EMAIL').asEmailString()).to.be.a('string')
+      expect(mod.get('EMAIL').asEmailString()).to.equal(TEST_VARS.EMAIL)
+    })
+
+    it('should throw due to a bad email address', function () {
+      process.env.EMAIL = '.invalid@example.com'
+
+      expect(() => {
+        mod.get('EMAIL').asEmailString()
+      }).to.throw('env-var: "EMAIL" should be a valid email address')
+    })
+  })
+
   describe('#example', () => {
     let fromMod