Use BuildEnvironment to parse cross-references
* Cross-reference data is parsed from Python Domain and Standard Domain instead of nodes now
Resulting improvements
* Linkcode extension no longer needs to be set up
* Predictable target parsing for all cross-references
* Support for any cross-reference role in the standard domain (:doc:, :ref:, :confval:, etc)
* Support for cross-refs with explicit titles
* Support for cross-ref/toctree hyperlinks containing inline literals
**Full Changelog**: https://github.com/TDKorn/sphinx-readme/compare/v0.0.2...v0.1.0