Add the templated function TomlDocument::get<T>()

Allows reading a value directly, without having the result to error
checking. A default value of 0 is used when the key is not found.

Example usage:
const int vali = doc.get<int>("section.subsection.target");
const double vald = doc.get<double>("section.target");

Note: The specialization TomlDocument::get<std::string>(); is missing.
Add the templated function TomlDocument::get_array<T>()

Allows reading an array directly, without having the result to error
checking. An empty std::vector<T> is used when the key is not found.

Example usage:
const std::vector<int> vali = doc.get_array<int>("section.subsection.target");
const std::vector<double> vald = doc.get_array<double>("section.target");

Note: The specialization TomlDocument::get<std::string>(); is missing.
Merge pull request #10 from nbigaouette/template_get

New templated functions