Consider adding a query for site title in Header

[josefaidt]

Description

Adding a query for the site title defined in gatsby-config ensures an easier adaptation to getting started with the theme's starter rather than manually changing it in two places. Not sure if this was the intended functionality, but if so I can close this issue. :)

Moving Forward

If this is something ya'll would like to include I have a PR ready to submit as I made this change in my application.

[josefaidt]

// src/gatsby-theme-carbon/components/Header.js
import React from 'react'
import { useStaticQuery, graphql } from 'gatsby'
import Header from 'gatsby-theme-carbon/src/components/Header'

const CustomHeader = props => {
  const query = useStaticQuery(graphql`
    query SITE_TITLE {
      site {
        siteMetadata {
          title
        }
      }
    }
  `)
  return (
    <Header {...props}>
      <span>{query.site.siteMetadata.title}</span>
    </Header>
  )
}

export default CustomHeader