[gatsby-theme-blog-core] Support filter, limit as theme options

[Owen]

Hi,

Here is the case, I want exclude the draft: true posts and custom posts limit, but now I can not do that.

Expect:

{
      resolve: `gatsby-theme-blog-core`,
      options: {
        filter: {
          frontmatter: { draft: {ne: true} }
        },
        limit: 500
      },
    },

I can make a pr later.