Bubbling of nested directives

[SomMeri]

Related to #1691 and #2104

Directives are changed to work the same way as media. The biggest code change is that the rules property of directive.js contains an array instead of just a ruleset.

Directive bubbling ends at the closest directive or media, e.g. this remains unchaged by compilation:

@media print {
    @supports (upper: test) {
        @supports not (-webkit-font-smoothing: subpixel-antialiased) {
           @media screen {
              nested {
                property: value;
              }
           }
        }
    }
}

[SomMeri]

Travis is failing, but other less.js pull requests are failing on the same error.

[lukeapage]

Please could you merge latest? then builds should pass...

[lukeapage]

great work though, thanks, will be a nice feature

[SomMeri]

I merged it and build is passing. You are welcome ;)