Fixes#33.
This PR uses cmake's interface library feature:
An INTERFACE library target does not directly create build output,
though it may have properties set on it and it may be installed,
exported and imported.
This makes it easier to include the header only library in a cmake
project. After using add_subdirectory on the cxxopts directory, one
simply needs to include cxxopts in their target_link_libraries, which
will allow the user's target to inherit the properties of the cxxopts
header library (see changes to example and test).
