Doorstop

- Fixed a bug checking levels across inactive items.
- Added error message for all IO errors.
- Added '--skip' options to disable documents during validation.
- Added Mercurial support. ([tjasz](https://github.com/tjasz))

- Restrict `openpyxl < 2.2` (there appears to be a breaking change).

- Fixed a bug running VCS commands in subdirectories.
- Excluded `openpyxl == 2.1.0` as a dependency version.

- Limit the maximum version of `openpyxl` to 2.1.0 due to deprecation bug.

- Fixed a bug requesting new item numbers from the server.

- Added `doorstop clear ...` to absolve items of their suspect link status.
- Added `doorstop review ...` to absolve items of their unreviewed status.
- Added `Item.clear()` to save stamps (hashes) of linked items.
- Added `Item.review()` to save stamps (hashes) of reviewed items.
- Added `doorstop reorder ...` to organize a document's structure.
- Renamed `Item.id` and `identifer` arguments to `uid`
- Added '--no-body-levels' to `doorstop publish` to hide levels on non-headings.
- Added `doorstop-server` to launch a REST API for UID reservation.
- Added '--server' argument to `doorstop add` to specify the server address.
- Added '--warn-all' and '--error-all' options promote warnings to errors.