Tox-poetry-installer

- Add support for the Tox
[`extras`](https://tox.readthedocs.io/en/latest/config.html#conf-extras) configuration
parameter
- Update runtime-skip-conditional checks to improve clarity and ease of future maintenance
- Update lockfile parsing to avoid parsing it multiple times for a single testenv
- Fix missing `poetry-core` dependency when using Poetry\<1.1.0

- Fix support for Poetry-1.1
([2](https://github.com/enpaul/tox-poetry-installer/issues/2))
- Include tests in sdist ([8](https://github.com/enpaul/tox-poetry-installer/issues/8))

- Fix usage of the plugin in non-Poetry based projects
([1](https://github.com/enpaul/tox-poetry-installer/issues/1))
- Fix treating dependency names as case sensitive when they shouldn't be
([7](https://github.com/enpaul/tox-poetry-installer/issues/7))

- Fix breaking when running Tox in projects that do not use Poetry for their
environment/dependency management
([1](https://github.com/enpaul/tox-poetry-installer/issues/1))

- Fix duplicate installation of transient environment dependencies
- Fix logging error indicating all environments always have zero dependencies
- Fix installing main dependencies when `skip_install` is false but `skipdist` is true

- Add support for per-environment configuration setting `require_locked_deps`
- Add support for per-dependency lock requirement setting using `poetry` suffix
- Add support for coexisting locked and unlocked dependencies in a single test environment
- Update documentation to include more usage examples
- Update documentation to improve clarity around problems and drawbacks
- Fix logging messages being inconsistently formatted
- Fix raising the same exception for "locked dependency not found" and "locked dependency
specifies alternate version" errors
- Fix plugin errors not reporting to Tox that they happened
- Fix plugin errors not causing Tox to mark the env as failed