`init` should create default tables

[jamesaoverton]

nanobot init should create three default tables for VALVE in the src/schema/ directory:

• table.tsv lists all the tables that Nanobot knows about
• column.tsv lists all the columns of all the tables
• datatype.tsv lists all the datatypes used by the columns

These three tables are used to validate themselves. I added these to the default_tables branch.

Once the three TSV files are created, Nanobot should run VALVE to load them into a SQLite database. So we'll need to add a dependency: ontodev_valve = "0.1.12".

By default the SQLite database should be created in the project root, and it should be called .nanobot.db. There should be a configuration option for the path to this file.

If there's a .gitignore file, then /.nanobot.db should be appended to it (unless it's already present).

[jamesaoverton]

The user's steps for creating a new project would be something like:

$ mkdir new_project
$ cd new_project
$ nanobot init
$ tree
├── nanobot.toml
├── .nanobot.db
└── src
    └── schema
        ├── column.tsv
        ├── datatype.tsv
        └── table.tsv