Add TlbExp option to build targets

[govert]

For add-ins that expose the COM Server, there is an additional step in the build process, to run TlbExp to generate a .tlb file from the compiled .dll.

It would be nice to have that as an option in the build process.

[augustoproiete]

Example of usage w/ post-build events, for reference: https://groups.google.com/forum/#!topic/exceldna/e9bZBrfGfaQ

[govert]

Some hints on dealing with the output paths here: https://stackoverflow.com/questions/60191045/msbuild-how-may-i-access-built-in-properties-while-defining-my-own-custom-prope