#2 Allow other extensions for src/tgt files

[Tamara Harpster]

With the initial release of HTMLInc, the file
extensions for the source and target files are hard
coded into the program. This should be changed so that
the user can specify what extensions can be used.
Also, another enhancement would be including some
intelligence for knowing different extensions for
comments for the includes and being able to specify
the main tag for the include mention in the source
files.