Add user-defined literal for deserialization from character sequences

[fktn-k]

It is useful for users of the fkYAML library to implement the user-defined literal for deserialization from character sequences.
Supported character types are char, char16_t and char32_t just as the currently supported input character types.
Note that std::FILE* and std::istream objects cannot be passed to the user-defined literal described above; use fkyaml::operator>>() function regarding such types.

[fktn-k]

Completed in #235.