Add support for array operations in the dsl.

[Aaronepower]

It'd be nice to if the dsl supported array operations such as append or remove.

Rust

struct Post {
     id: i32,
     authors: Vec<String>,
}

fn main() {
    use schema::posts::dsl::*;
    diesel::update(posts.find(&1))
      .array_append(authors, String::from("Jane Doe"));
}

SQL

UPDATE posts SET authors = authors || '{Jane Doe}' WHERE id = 1;

Checklist

• I have already looked over the issue tracker for similar issues.

[sgrif]

Note that your SQL doesn't map to the operator or types you've specified. || is the "concatenate" operator, and the equivalent Rust would likely be authors.eq(authors.concat(vec!["Jane Doe"]))