Need a '%filename%' expansion for htmlTag

[malaney]

Currently the htmlTag supports a %path% variable expansion to allow the full path of a processed asset (i.e. /assets/compiled/logo.gif) to be placed within the output that gassetic renders. It would also be nice to add a %filename% expansion that allows us to inject only the "original" base filename of the processed asset (i.e. logo.gif) into the output. In this case we would want the original filename (logo.gif) and not the post-processed filename (logo_0.gif) so that we could reference the original filename as needed.

Use case:

htmlTag: '

gassetic should render this as ...

In this way we get full image revving with a reference to the original filename.