Allow unparenthesized expressions to contain more than one comparison operator

[andrew-skybound]

Ruby SASS allows multiple comparison operators to appear in a single unparenthesized expression, in the format:

    a == b == c
    a > b == c

And so on. Indeed "c" needs to be a boolean value for this to be useful but it is nevertheless supported. Libsass currently only allows one comparison operator (== != > >= < <=) per unparenthesized expression.

Input:

$x: 2;
$y: 1;

@function getResult() {
    @return true;
}

.test {
    a: $x > $y == getResult();
}

Ruby SASS Output (3.4.19):

.test {
  a: true;
}

Libsass Output (3.2.5):

Error: error reading values after $y
        on line 9 of test.scss
>>      a: $x > $y == getResult();
   -----------^

[xzyfer]

Spec added sass/sass-spec#603

[mgreter]

This was one of the major fixes in the milestone 3.4 branch and I've added a pretty extensive test suite to back up this feature! Code for it is a bit dirty in some places, but specs do pass!