feat(plugins): run verifications in series

With this new feature you can pass an array of plugin locations/names to
both "verifyConditions" and "verifyRelease" in your `package.json`'s "release"
field. This will run multiple verification plugins in series.

test/mocks/plugin-error-a.js

@@ -0,0 +1,3 @@
+module.exports = function (config, options, cb) {
+  cb(new Error('a'))
+}

test/mocks/plugin-error-b.js

@@ -0,0 +1,3 @@
+module.exports = function (config, options, cb) {
+  cb(new Error('b'))
+}

test/mocks/plugin-result-a.js

@@ -0,0 +1,3 @@
+module.exports = function (config, options, cb) {
+  cb(null, 'a')
+}

test/mocks/plugin-result-b.js

@@ -0,0 +1,3 @@
+module.exports = function (config, options, cb) {
+  cb(null, 'b')
+}