SQL doesn't have a string type, and interpreting 'string' as text is actually contrary to at least SQLite3's behavior: "Note that a declared type of "STRING" has an affinity of NUMERIC, not TEXT." http://www.sqlite.org/datatype3.html
Add support for char(n) types via the new `:fixed_string` column type.

Example:

        add_column :people, :zip, :fixed_string, limit: 5