Add TSV writer and update other writers' docs

[cthoyt]

Closes #106

1. Adds more docs to JSON-LD export, which supports MONDO Robot use case
2. Adds more docs to SHACL export
3. Implements simple TSV export, which supports SemanticSQL use case
4. Adds section "writing a context" into the tutorial that links to the writer functions

cc @joeflack4