Flint 1.0.0 Release Candidate 1

- [x] Update `custom-header.php` to give more accurate preview
- [x] Update color scheme
 - Update screenshot.png
- [x] Add text domain `'flint'` where appropriate
- [x] Change theme tags to match WordPress standards
- [x] Add `editor-style.css`
- [x] Add featured image support
- [x] Pass WordPress Theme Check