add option to load default values about toml11

[ToruNiina]

Hello, Thomas.

It sounds nice.
I added toml::get_or() function to be able to load default values.

toml::Table data; // empty table!

const auto value1 = toml::get_or(data, "key1", 42); // value1 => int 42.

toml::Integer i(123);
const auto value2 = toml::get_or(data, "key1", i);  // value2 => toml::Integer 123.

Since toml::Table is just an alias of unordered_map<key, value>, it is not possible to add member function like table.at_or("key", optval).

toml::get_or internally call toml::get<T> if the table has the value corresponding to the key.
The type T for toml::get is deduced from the type of the optional value.

Best,
Niina