Add sentence about U+0000 to spec

Fixes #212.
Off-by-one error in utf8proc_detab
Validate UTF-8 input

Invalid UTF-8 byte sequences are replaced with the Unicode replacement
character U+FFFD. Fixes #213.
Add test for input containing null character
Merge pull request #217 from nwellnhof/utf8_validation

UTF-8 validation