Option to preserve match alignment

Some people like to align match arms for readability

match val {
    &TOMLValue::Integer(_)        => ArrayType::Integer,
    &TOMLValue::Float(_)          => ArrayType::Float,
    &TOMLValue::Boolean(_)        => ArrayType::Boolean,
    &TOMLValue::DateTime(_)       => ArrayType::DateTime,
    &TOMLValue::Array(_)          => ArrayType::Array,
    &TOMLValue::String(_,_)       => ArrayType::String,
    &TOMLValue::InlineTable(_)    => ArrayType::InlineTable,
    &TOMLValue::Table             => panic!("Cannot have a table in an array"),
 }

And currently it gets reformatted like this

match val {
    &TOMLValue::Integer(_) => ArrayType::Integer,
    &TOMLValue::Float(_) => ArrayType::Float,
    &TOMLValue::Boolean(_) => ArrayType::Boolean,
    &TOMLValue::DateTime(_) => ArrayType::DateTime,
    &TOMLValue::Array(_) => ArrayType::Array,
    &TOMLValue::String(_, _) => ArrayType::String,
    &TOMLValue::InlineTable(_) => ArrayType::InlineTable,
    &TOMLValue::Table => panic!("Cannot have a table in an array"),
}

It would be nice if rustfmt offered an option like match_align_arms that can have multiple values:

• Always: rustfmt will try to align all (non-block) match arms
• Preserve: rustfmt will preserve alignment if it is already aligned manually. For matches that are not already aligned, current behavior will be used.
• Never: current behavior

I think Preserve should be the default behavior.

/cc @joelself

Use the aligned and currently reformatted match-arm examples as the behavioral reference. Locate rustfmt’s match-arm formatting and configuration handling, then determine how Always, Preserve, and Never should interact with existing behavior. Done means the option semantics are implemented and covered by formatting tests, though this issue names no specific files or tests.