Move implementation into cpp

[psyklopz]

Addresses issue #9

When Configuru is included in separate translation units, it can result in LNK2005 errors on MSVC.
GCC seems to handle this fine, but it is still a violation of the One Definition Rule, where it appears Configuru is still built into multiple translation units.

This commit is similar to that done on Loguru two years ago.

I have shown this works successfully in two different projects that utilize Configuru in separate units. One uses two different configuration files. The other uses Configuru for a config file and then some simple JSON formatting for a web API.

I don't use Boost and really didn't feel like messing with getting that to work. So I was able to build the test suite (using std::experimental::filesystem instead of boost::filesystem) and confirm the tests still all pass. I've not included this in the merge request, but if interested the code that works with std::experimental::filesystem can be seen on the separate_into_hpp_and_cpp branch of psyklopz/Configuru.

[emilk]

I think this is an excellent idea, but we no longer need CONFIGURU_IMPLEMENTATION. In fact, its use is indicative of a user error - someone updated Configuru and did not realize they now need to make use of configuru.cpp.

[emilk]

Here it would be good with this check:

#if defined(CONFIGURU_IMPLEMENTATION)
    #error "You are defining CONFIGURU_IMPLEMENTATION. This is for older versions of Configuru. You should now instead include configuru.cpp (or build it and link with it)"
#endif

[emilk]

We do not need CONFIGURU_IMPLEMENTATION anymore!

[psyklopz]

I will make these changes. I'm a little swamped at the moment, so it probably won't be until next week.