Doorstop

- Fixed missing `six` dependency.

- Updated dependencies.

- **BREAKING:** Switched to SHA256 for tracking changes to files and reviews of extended attributes. Use `doorstop review all` followed by `doorstop clear all` to update an existing project. WARNING: This marks all items as reviewed and clears all suspect links.
- Dropped support for Python 3.5.
- Fixed a bug with items not getting saved after edits.
- Added the ability to explicitly name a requirement.
- Improved the error message when there are multiple root documents.

- Updated `edit` and `reorder` commands to use `$EDITOR`.
- Promoted support for item headers out of beta.
- Fixed bug with Doorstop attempting to import formulas from XLSX.
- Added support for `!include` statements in YAML to share configuration.
- Added `--default` option to `add` to specify a file with defaults values.

- Added preliminary support for item headers. ([rickeywang](https://github.com/doorstop-dev/doorstop/pull/285))
- Added major enhancements to the Desktop GUI. ([elarivie](https://github.com/doorstop-dev/doorstop/pull/290))
- Converted document outline to a tree view.
- Made documents navigable via clicking links.
- Added an icon for Linux and Windows.
- Fixed duplicate headings when publishing. ([guille-r](https://github.com/doorstop-dev/doorstop/pull/302))
- Added Python 3.7 support.

- Fixed issue running doorstop on CI. ([ojohnny](https://github.com/doorstop-dev/doorstop/pull/281))