Add arbitrary byte escape sequence.

Only having a way to insert a null byte into a string seems shortsighted. This introduces a syntax to allow insertion of arbitrary bytes into a string.

```
\xXX - byte            (0x00-0xFF)
```

Where each `X` is a hex digit. Examples:

```toml
name      = "Jos\x82"             # "José" in latin1 encoding.
binary_ip = "\x0A\x00\x00\x01"    # 10.0.0.1 as a four byte binary.
```