add find() and find_as<T>()

`find()` and `find_as()` behave almost the same as `get()` and `get_as()` - the only difference is that `find*()` follows _qualified keys_. Qualified keys are the full access path joined with dots. 

    [foo.bar]
    baz = "nested"
    # qualified key for baz is: foo.bar.baz

And with `find*()` one can access the value from above like:

    auto config = cpptoml::parse_file(/* ... */);
    auto baz = config.find_as<std::string>("foo.bar.baz");

I've added two new methods, because I'm not sure if we should support this feature within `get*()` directly. To me it sounds like a good idea but would require some changes on methods like `contains()` too. 

At least this is a working feature and the decision is up to you (introduce `find*()` or merge it into `get*()` and others) :)