Add extra navigation tests for #318
Fix test for #318
Support custom intro node, fixes #318
Add test for level regarding #318
Refactor Summary rendering

Introduction is no longer an edge case
Adapt summary tests to new summary parsing

We now insert default README.md (introduction) node in parsing the
SUMMARY.md logic
Remove special case in navigation

For intro node
Normalize intro node in Summary parsing
Merge pull request #319 from GitbookIO/enchancement/custom_intro

Enchancement/custom intro