Copy generated help file to publish directory

When we create a packed add-in (or an unpacked add-in) in the publish directory, we should also copy the generated .chm file there.
Maybe we can add a build target to the ExcelDnaDoc that checks what the properties of the add-in build are, and if appropriate copy the .chm to the right 'publish' directory.

For version 1.7.0 I don't expect to pack the .chm file into the .xll file, though I imagine a future version might (it would need some help in Excel-DNA during the function registration to use the temp path to an extracted help file).