New templated functions

Add two new templated functions TomlDocument::get<T>() and TomlDocument::get_array<T>().

This simplifies greatly the reading of values from the toml file. Since previous get() returns a kind of pointer, one needs to verify if it is null or not before trying to extract the actual value by casting to the right type.

Instead, these templated functions will do all the check themselves and will return the expected value of the right type.

Note that both functions don't work yet for std::string types.