Add configurable header alignment - #8

Summary

• add column VerticalAlign plus HeaderAlign and HeaderVerticalAlign configuration
• inherit column alignment in each header unless an explicit header override is set
• apply the same behavior to generated templates
• set the package version to 0.6.1-prerelease and document the API

Verification

• dotnet test tests/CellSharp.Tests/CellSharp.Tests.csproj --no-restore (227 passed)
• dotnet build src/CellSharp/CellSharp.csproj --no-restore (net8.0 and netstandard2.0)