curly: Add "as-needed" option

[andy-hanson]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Adds the never option, which forbids unnecessary curly braces.

CHANGELOG.md entry:

[new-rule-option] curly: "as-needed" option

[ajafff]

There's one edge case that needs to be handled. You can't remove the curly braces in the following example

if (foo) {
    if (bar)
        foo(bar);
} else
    bar();

[adidahiya]

I see that this option name is similar to trailing-comma and space-before-function-paren, etc. but I think naming it "as-needed" instead of "never" would be less confusing & closer to the true intention of its implementation