Add marked-text-renderer@0.0.1 dependency
Move page lexer to separate module

Allow lexer to be used independently of page parser/renderer
In page parser, lex only if not already lexed
Add lunr@0.5.2 dependency
Generate search_index.json when generating site

Fixes #24
Generate section.id in lexer

This allows generating correct URLs for our search index
Merge pull request #39 from GitbookIO/feature/search

Feature/search