Add support for preserve_single_line_if::Bool

[alexandergunnarson]

Before formatting (or with preserve_single_line_if = true):

function f()
    if rand() < 0.5 return 2 end
    0
end

After formatting with preserve_single_line_if = false (default):

function f()
    if rand() < 0.5
        return 2
    end
    0
end

Resolves #516.

[domluna]

please add some test cases

looks good otherwise.