Add option to lit to embed referenced files into wixlib

Here's the short version:

Add an option to lit that would cause external files
referenced by the wixobj files to be embedded within the
produced wixlib file.

Here's the extended scenario that describes the
justification:

We have a situation where multiple development
projects depend on the build output from a common,
shared development project. Each of these development
projects has its own unique installer (i.e., they are truly
distinct products) that, in addition to installing its own
files, must install the files from the shared components.

Currently, there seem to be two options to accomplish
this:
1. Use a merge module for the shared components.
2. Use WiX fragments/libraries for the shared
components.

Using a merge module has the benefit that the output
from the shared development project is a single msm
file that can be referenced from the other installers (and
checked into source control within those projects).

However, using a merge module offers less flexibility in
designing the overall installer approach--we would rather
cause the output from the shared project to be a single
wixlib file that would be referenced from the other
projects. Unfortunately, this means that all of the files
referenced by the wixobj files that went into the wixlib
must also be part of the output from the shared project,
and must be checked into the other projects' source
control systems. To minimize the pain, our build
process for the shared project gathers up the referenced
files into a zip file--this way there's only two files that
make up the output from the shared project: a wixlib file
and a zip file. Of course, this slightly complicates the
build process for the other projects.