document and test (on travis/appveyor) how to find Blitz++ library from CMake

[slayoo]

Just for the record, if you only want to find Blitz++ library from CMake, this works:

find_package(PkgConfig)
pkg_check_modules(BLITZ QUIET blitz>=0.10)
if (NOT BLITZ_FOUND)
    ...

Originally posted by @slayoo in https://github.com/blitzpp/blitz/issues/4#issuecomment-174218649