Support Gatsby plugins with resolve

[znarf]

In the following file, https://github.com/badges/shields/blob/master/gatsby-config.js, gatsby-plugin-page-creator is not detected because it's set using a different syntax than what is currently expected.

 plugins: [
    {
      resolve: 'gatsby-plugin-page-creator',
      options: {
        path: `${__dirname}/frontend/pages`,
      },
    },
    'gatsby-plugin-react-helmet',
    'gatsby-plugin-catch-links',
    'gatsby-plugin-styled-components',
    'gatsby-plugin-remove-trailing-slashes',
    'gatsby-plugin-typescript',

[nagygergo]

Also, this is a valid config too, so resolving the used plugins seems to be a harder task:

 'gatsby-plugin-sharp',
'gatsby-transformer-sharp',
{
      resolve: 'gatsby-transformer-remark',
      options: {
        plugins: [
          {
            resolve: 'gatsby-remark-relative-images',
            options: {
              name: 'uploads',
            },
          }]
     }
}

[nagygergo]

Gatsby identifies the following elements as plugin entries:

• gatsby-plugin-sharp
• { resolve: 'gatsby-transformer-sharp', options: {} }
• { resolve: 'gatsby-transformer-remark' }

Also there is an option for plugins to have plugins of their own. So the based on the above snippets I'd consider the following rules to find the used plugins:

• Plugin entry must be an element of an array that is under a plugins key. So plugins: [<pluginEntry>].
• Plugin entry must be one of the above shapes above.