Add a framework for issuing deprecation messages (#429)

[KyleFromKitware]

This adds two new commands, tribits_deprecated() for issuing generic deprecation messages, and tribits_deprecated_command() for deprecating functions and macros.

Addresses #429

[bartlettroscoe]

@KyleFromKitware, thanks for adding this PR so fast! In addition to the suggestions below, I have just a few other requests:

• Please add entries for these new functions to the file tribits/doc/guides/UtilsMacroFunctionDocTemplate.rst (which is my poor-man's Doxygen for CMake code).'

• Please add the cache var TRIBITS_HANDLE_TRIBITS_DEPRECATED_CODE for the TriBITS framework to the (large) macro tribits_define_global_options_and_define_extra_repos() (We will pull that macro out into its own module at some point here and even break it up into multiple modules depending on how we want to componentize TriBITS as per Exposing parts of TriBITS as CMake modules #368.)

[bartlettroscoe]

@KyleFromKitware, thanks for the changes! I think this PR is good to merge.