Expose `$mixins` var within template functions

Within code blocks in your template code, you can now access Jade mixin functions via the $mixins variable. In virtual-jade, mixins boil down to functions that take arguments and return a tree of h(name, attrs, children). They are like React stateless components. Accessing them via $mixins is useful for special cases where you want to pass around handles to blocks of Jade code as callback functions, e.g.:

mixin item(x)
  .item
    .more-tree= x + 1

list-virtual-scroll(props={itemRenderer: $mixins.item})

// in list-virtual-scroll.jade
each val in allItems.slice(startIdx, endIdx)
  = props.itemRenderer(val)