docs: Custom syntax map

[youtiao66]

By default CSScomb process *.css, *.less, *.sass and *.scss files in matched syntax.
You can custom syntax map by syntax config. e.g. Using css syntax to handle *.wxss files

{
    "syntax": {
        ".wxss": "css"
    }
}

WXSS (WeiXin Style Sheets) is very similar to CSS but a little different