feat: set tag author and committer name/email

Use [Git Environment Variables](https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables#Committing). Set default values if environement variables are not set.

lib/definitions/constants.js

@@ -0,0 +1,9 @@
+const RELEASE_TYPE = ['major', 'premajor', 'minor', 'preminor', 'patch', 'prepatch', 'prerelease'];
+
+const FIRST_RELEASE = '1.0.0';
+
+const COMMIT_NAME = 'semantic-release-bot';
+
+const COMMIT_EMAIL = 'semantic-release-bot@martynus.net';
+
+module.exports = {RELEASE_TYPE, FIRST_RELEASE, COMMIT_NAME, COMMIT_EMAIL};