Support for `insert or ignore` for Sqlite.

[alatiera]

Atm you kinda have to isolate the query and throw away the result in order to do that.

let _ = diesel::insert(foo)
    .into(atable)
    .execute(con);

It would be nice if you could do something like this:

diesel::insert_into(atable)
    .values(foo)
    .on_conflict()
    .do_nothing() 
    .execute(con)?;

Or even on_conflict().update() run replace_into instead?

I think this sort of functionality exists for Postgres with .on_conflict().do_nothing() but It would be nice if something like that existed for Sqlite too.