Text Search

[iddan]

MVP implementation of text search based on Bleve. There are many capabilities not included in this PR but anything beyond the basic text search should be added in additional PRs.

Resolves #428

Tasks

• [ ] For KV stores, Implement Bleve's KV Store instead of creating a file.
• [x] Update data model to create single property Bleve documents
• [x] Hook up search.Index() and search.Delete() for quads write and delete
• [x] Hook up search configuration to Cayley configuration
• [x] Hookup search.Configuration to Cayley config file (and CLI?)
• [ ] Documentation:
  • [x] Tutorial
  • [ ] Detailed documentation of configuration options

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This change is 

[iddan]

Bleve will only be implemented for in-memory and key-value backends for now.