Enable custom path manipulations in copy function.

[asmagin]

I cannot use EJS templates in path or body of files, as I'm building complex templates for Visual Studio projects. I need to be able to run C# project to modify or update it.

Content replacement can be done via regex, but EJS templates are built in copy() and I cannot move avoid using them in current implementation.

There is a copyTpl() function that provide templates implementation via ejs, while copy() must remain generic and extendable, allowing to pass custom functions to process body and path of the file.