prepare for 0.10.0 release

Merge #90

90: Allow extensions on the root element r=urschrei a=lnicola

- [x] I agree to follow the project's [code of conduct](https://github.com/georust/geo/blob/master/CODE_OF_CONDUCT.md).
- [x] I added an entry to `CHANGELOG.md` if knowledge of this change could be valuable to users.
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Fixes #90

Co-authored-by: Laurențiu Nicola <[email protected]>

Merge #88

88: Drop authors entry from manifest r=lnicola a=lnicola

- [x] I agree to follow the project's [code of conduct](https://github.com/georust/geo/blob/master/CODE_OF_CONDUCT.md).
- [x] I added an entry to `CHANGELOG.md` if knowledge of this change could be valuable to users.
---

This is soft-deprecated: https://rust-lang.github.io/rfcs/3052-optional-authors-field.html.

Co-authored-by: Laurențiu Nicola <[email protected]>

Bug fixes for spec compliance, MSRV 1.56

Custom event writer, serde, track extensions

Allow empty strings in trk names

Correct desc tag parsing, add copyright tag parsing

prepare for 0.8.1 release

prepare for 0.7.0 release

Prepare for 0.6.0 release.