Support opting out of the default [error] prefix

[ken-matsui]

toml11 prints error messages with the [error] prefix by default.
Defining TOML11_NO_ERROR_PREFIX will let toml11 omit the prefix regardless of colorized or not in case you would use a custom prefix, such as Error:.

#define TOML11_NO_ERROR_PREFIX

---

Without TOML11_NO_ERROR_PREFIX:

[error] value should be positive
 --> example.toml
 3 | num = -42
   |       ~~~ positive number required

With TOML11_NO_ERROR_PREFIX:

value should be positive
 --> example.toml
 3 | num = -42
   |       ~~~ positive number required

We are now able to customize the prefix like:

fmt::print("Error: {}", e.what());

This prints out:

Error: value should be positive
 --> example.toml
 3 | num = -42
   |       ~~~ positive number required

[ToruNiina]

This is a useful feature. Thank you!