Add Elvis/null coalescing operator rule

[CosmeValera]

This issue is a mention to this comment in the accepted PR of @JesusValera . I will quote:

I was thinking, maybe it makes sense to also add this rule for the Elvis/null coalescing operators?

// Not this
$foo = 'foo' ??
    'bar';

$foo = 'foo' ?:
    'bar';

// But this
$foo = 'foo'
    ?? 'bar';

$foo = 'foo'
    ?: 'bar';

I could add the modifications in this (or another PR), let me know what you think and if you have another suggestion 😄

Originally posted by @JesusValera in #93 (comment)

[CosmeValera]

My bad, it is already done in https://github.com/php-fig/per-coding-style/pull/93/files?diff=unified&w=0