hide property begin with $

I want to make use of property lookup to implement some powerful mixins which looks for parent block's property. e.g a dynamic semantic grid mixin
```
grid-ctn(containerwidth, col, padwidth = false)
  $grid_width grid-devide(containerwidth, col)
  $grid_col col
  padwidth = grid-devide(containerwidth, col * 8) unless padwidth
  $grid_padding grid-devide(padwidth, 2)

  width containerwidth
  position relative
  clearfix()

grid-block(colspan)
  float left
  width (colspan * @$grid_width) - 2 * unit(@$grid_padding, '')
  margin-left @$grid_padding
  margin-right @$grid_padding

grid-pull(colspan)
  margin-right colspan * @$grid_width + unit(@margin-right, '')

grid-push(colspan)
  margin-left colspan * @$grid_width + unit(@margin-left, '')

grid-devide(width, division)
  u = unit(width)
  num = unit(width, '')

  if u is '%'
    unit(floor(num / division * 1000) / 1000, u)
  else
    unit(num / division, u)
```
so I can easily define a grid, where `grid-block` mixin would look for `$grid_xxx` property to decide its width&margin
```
.container
  // grid-ctn 960px 12 5px
  grid-ctn 100% 12
  .halfblock
    grid-block 6
    &.center
      grid-push 3
```

this works great except for the extra `$xx` property output, so I think hide $hidden property could be nice

inspired by #1014