Variable Extrude an OpenScad Library: linear_extrude with function driven scaling

##Variable Extrude

**About this Library**
This is an implementation of linear_extrude, where the scale parameter can also be a function.
With the exeption of the scale parameter this works like the built in [linear_extrude](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Using_the_2D_Subsystem#linear_extrude)(the v parameter from the snapshot version is not implemented).

**Usage**
*1. Add to your library*
1. Copy the `variable_extrude.scad` to your scad library folder.
2. add "`use;`" in your scad file
3. use `variable_extrude` with same parameter as [linear_extrude](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Using_the_2D_Subsystem#linear_extrude) (exept v)

*2.How to use the scale paramter*
Like the original [linear_extrude](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Using_the_2D_Subsystem#linear_extrude) the scale parameter exepts either a scalar(`single_value`) or a vector(`[x_value,y_value]`). In this implementation the values can either be a numbe