Also adds `Predicate` helper function to create `PredicateMatcher`.
Because of limitations in type inference it needs to be explicitly
typed, like so
`Predicate<std::string>([](std::string const& str) { ... })`.
It also takes an optional second argument for description of the
predicate.

It is possible to infer the argument with sufficient TMP, see
https://stackoverflow.com/questions/43560492/how-to-extract-lambdas-return-type-and-variadic-parameters-pack-back-from-gener/43561563#43561563
but I don't think that the magic is worth introducing ATM.

Closes #1236

The old version would lead to error when Catch was installed
as a subproject. The file would be written to the subproject's
build directory and then would not be installed properly.

Closes #1158

Create a namespaced Catch2::Catch target that is 'linkable' through
`target_link_libraries()` and export it so it is findable through
`find_package()`.

`find_package()` will find versions with the same major number and with
minor number >= requested.

This makes catch a lot easier to use in CMake-based projects. Whether it
is found using `find_package` or included in the client project as a
subdirectory, the client can include the catch headers per-target with
`target_include_directories(target PRIVATE Catch2::Catch).

Example usage:

    cmake_minimum_required(VERSION 3.1)

    # include Catch2 as subdirectory or installed package
    # add_subdirectory(Catch2)
    find_package(Catch2 VERSION 2.1.0 REQUIRED)

    add_executable(tests tests/catch_main.cpp)
    target_link_libraries(tests PRIVATE Catch2::Catch)