Add PRODUCT_PUBLISHER as an NSIS variable

[breisfeld]

Within the local AppDir on windows, the application author or publisher is often included as the parent path of the product, for example C:\Users\username\AppData\Local\publisher_or_author\appname. Thus, exposing the publisher, would allow installation into this hierarchy via something like the following in the pynsist config file:

filepath > $LOCALAPPDATA/${PRODUCT_PUBLISHER}/${PRODUCT_NAME}