feat: allow to disable the publish plugin hook

`verifyConditions` explanation was wrong previously.

docs/usage/plugins.md

@@ -91,7 +91,7 @@ For example:
 With this configuration:
 - the `custom-plugin` npm module will be used to [analyze commits](#analyzecommits-plugin)
 - the `./build/my-plugin.js` script will be used to [generate release notes](#generatenotes-plugin)
-- the [`@semantic-release/exec`](https://github.com/semantic-release/exec),  [`@semantic-release/npm`](https://github.com/semantic-release/npm) and [`@semantic-release/exec`](https://github.com/semantic-release/exec) plugins will be used to [verify conditions](#verifyconditions-plugin)
+- the [`@semantic-release/exec`](https://github.com/semantic-release/exec),  [`@semantic-release/npm`](https://github.com/semantic-release/npm) and [`@semantic-release/github`](https://github.com/semantic-release/github) plugins will be used to [verify conditions](#verifyconditions-plugin)
 - the [`@semantic-release/exec`](https://github.com/semantic-release/exec) plugin will be used to [verify the release](#verifyrelease-plugin)
 - the `cmd` option will be set to `verify-conditions.sh` only for the [`@semantic-release/exec`](https://github.com/semantic-release/exec) plugin used to [verify conditions](#verifyconditions-plugin)
 - the `cmd` option will be set to `verify-release.sh` only for the [`@semantic-release/exec`](https://github.com/semantic-release/exec) plugin used to [verify the release](#verifyrelease-plugin)

lib/definitions/plugins.js

@@ -45,7 +45,7 @@ module.exports = {
   publish: {
     default: ['@semantic-release/npm', '@semantic-release/github'],
     config: {
-      validator: conf => Boolean(conf) && (isArray(conf) ? conf : [conf]).every(conf => validatePluginConfig(conf)),
+      validator: conf => !conf || (isArray(conf) ? conf : [conf]).every(conf => validatePluginConfig(conf)),
     },
     output: {
       validator: output => !output || isPlainObject(output),

package.json

@@ -41,7 +41,7 @@
     "marked-terminal": "^3.0.0",
     "p-locate": "^3.0.0",
     "p-reduce": "^1.0.0",
-    "read-pkg-up": "^3.0.0",
+    "read-pkg-up": "^4.0.0",
     "resolve-from": "^4.0.0",
     "semver": "^5.4.1",
     "yargs": "^11.0.0"

test/definitions/plugins.test.js

@@ -60,9 +60,9 @@ test('The "prepare" plugin, if defined, must be a single or an array of plugins
 test('The "publish" plugin is mandatory, and must be a single or an array of plugins definition', t => {
   t.false(plugins.publish.config.validator({}));
   t.false(plugins.publish.config.validator({path: null}));
-  t.false(plugins.publish.config.validator());
 
   t.true(plugins.publish.config.validator({path: 'plugin-path.js'}));
+  t.true(plugins.publish.config.validator());
   t.true(plugins.publish.config.validator('plugin-path.js'));
   t.true(plugins.publish.config.validator(() => {}));
   t.true(plugins.publish.config.validator([{path: 'plugin-path.js'}, 'plugin-path.js', () => {}]));