added support for @ page rule

[fabianmieller]

Added support for @ page rule

if rule.type === 'page' then i added the @ page rule + the selectors.

i also added a case for type 'page'.

Results:

"@page": {
        "margin": "2cm",
},
"@page :left": {
        "margin-left": "4cm",
        "margin-right": "3cm"
},
 "@page :right": {
        "margin-left": "4cm",
        "margin-right": "3cm"
}

[HenriBeck]

I don't think we will accept this PR for now because @page rules aren't supported in jss yet

[kof]

we need to add @page support, we have an issue for that in jss repo