Support more string values for types in .add_column()

[simonw]

https://sqlite-utils.readthedocs.io/en/2.4.2/python-api.html#adding-columns says:

SQLite types you can specify are "TEXT", "INTEGER", "FLOAT" or "BLOB".

As discovered in #92 this isn't the right list of values. I should expand this to match https://www.sqlite.org/datatype3.html