fix: skip first ws/newlines in toml literal

when ""_toml literal is used with C++11 raw-string literal,
it normally starts with newline like the following.
```cpp
const auto v = u8R"(
    [table]
    key = "value"
    )"_toml;
```
With this, the error message shows the first empty line that starts just
after `u8R"(` and thus the error message shows nothing. To avoid this,
skip the first empty lines and whitespaces in literal.

toml/literal.hpp

@@ -73,6 +73,9 @@ inline ::toml::value operator""_toml(const char* str, std::size_t len)
     if(auto data = ::toml::detail::parse_toml_file(loc))
     {
         loc.reset(loc.begin()); // rollback to the top of the literal
+        // skip needless characters for error message
+        skip_line::invoke(loc); // skip the first several needless lines
+        skip_ws::invoke(loc);   // skip the first several needless whitespaces
         return ::toml::value(std::move(data.unwrap()),
                 ::toml::detail::region<std::vector<char>>(std::move(loc)));
     }