Add macro to set plural form for table name

[yanganto]

Make a macro or derive attribute to use plural form for the table name.
For example, here is using .person because the table relates to the person table.

SewUp/examples/rdb-contract/src/lib.rs

Line 105 in 075b64c

let owner = post.person()?;

SewUp/examples/rdb-contract/src/modules.rs

Line 18 in 075b64c

#[belongs_to(Person)]

When the relationship becomes one to many, or the plural wording is not regular,
we need a attribute to specify the plural form.