Add config for removing default WP plugins

[ckhicks]

It would be really handy to have a way to set "disallowed" plugins per-project. So if I wanted to remove Akismet from Wingstop, etc.

[chuckmo]

You can do this by adding the slug to the remove_plugins array in the wp_cli config file.

/grunt/configs/wp_cli.js, line 36:
remove_plugins: ['hello'],

[chuckmo]

CK commented elsewhere that it would be great to have this inside of the HP config object (as oppose to it's current place in the task config)

[chuckmo]

It would probably be simplest to just remove all of them and then re-install any that are desired via the usual plugin definition.