Prevent double configuration for the same plugin

[wooorm]

unified().use([[plugin, {foo: true, bar: true}], [plugin, {foo: false, qux: true}]]);

...should not attach the same plugin twice. Rather, the config should be merged as if the following was given:

unified().use(plugin, {foo: false, bar: true, qux: true});

This would allow, for example, some configuration for remark-lint to live in a preset, whereas other configuration is passed by the user:

unified().use({plugins: [
  require('remark-lint-preset-recommended'),
  [require('remark-lint'), {finalNewline: false}]
]});

Unfortunately it’s not possible to merge configuration from different .use calls:

unified().use(plugin, {foo: true, bar: true}).use(plugin, {foo: false, qux: true});

😒

[wooorm]

I made sure, in e73da90, that different .use calls also work!