(Partly) remove Grunt tasks

[swissspidy]

A scaffolded plugin contains a bunch of Grunt tasks which I think are unnecessary and can be removed or replaced.

• addtextdomain
  Note sure if that one is really needed. There was quite a discussion about that feature in the i18n-command repo. If anything, this could be a wp i18n addtextdomain command. (See https://github.com/wp-cli/i18n-command/issues/25)
• makepot
  People can just use wp i18n make-pot to generate POT files.
• wp_readme_to_markdown
  I don't see the point in this. If you host your plugin on WordPress.org, you need to adhere to the very specific format for readme.txt. If you host or develop your plugin on GitHub, you usually have notes for developers, information about the build status, etc. IMHO it makes more sense to have two separate files — readme.txt and readme.md with different information.

Finally, I don't see why Grunt is needed for these simple tasks. Why can't these just be simple scripts in package.json (e.g. "makepot": "wp i18n make-pot ." would be one such script)? One dependency less to worry about.

[schlessera]

I agree, we should look into getting rid of Grunt altogether. The tool itself is already pretty dated, and just including it for such simple stuff is a bad practice.