Consider adding writing functionality

[SimonGreenhill]

What would the API be?

Need to:

1. Generate metadata
2. identify/specify foreign keys
3. specify table types
4. add/remove tables.

# work off data.frames?
> write.cldf(table1, table2, table3, directory="xxx")

# work off a CLDF object?
> obj <- cldf()  # empty
> add_table(obj, name, table, type="wordlist")
> add_foreign_key(obj, 'a', 'b')
> make_metadata(obj)
> write.cldf(obj, directory="xxx")

[SimonGreenhill]

See #5