Allow else if

[ldemailly]

• else if in parsing
• pretty print them specially

if (i>=3) {10} else if (i>=2) {20} else {30}

initially parsed and printed as

if i >= 3 {
	10
} else {
	if i >= 2 {
		20
	} else {
		30
	}
}

which was correct but ugly, now

if i >= 3 {
	10
} else if i >= 2 {
	20
} else {
	30
}

🎉

fixes #87

[ldemailly]

this one is pretty cool to look at, it's only a few lines in parser, the rest is test and pretty print handling
(fyi async review @ccoVeille )