Reference an external assembly

[jowsy]

Is your feature request related to a problem? Please describe.
Allowing external references would allow you to use helper libraries in the code or third-part libraries.

Describe the solution you'd like
Implement the C#-scripting feature of referencing external assemblies using #r-directive.
The reference should be used for all cells in the current connection.

Following cell should add the reference for use in all cells.

#!revit
#r "c:\path\to\assembly.dll"