Add .tag folder handler

[Qix-]

It was decided upon to have a .tag/ folder, not dissimilar to .git/, that holds the state of the Tag system.

It would include:

• Dependency graphs/hashes for faster incremental builds (akin to Ninja's dependency files)
• Context configuration on a per-user basis
• Inheriting from configuration stored in ~/.tagrc, managed similar to how git config --global is managed