add option to treat warnings as errors

[vladak]

To catch problems early on, it would be handy to have an option to treat warnings as errors, similarly to the -Werror used by the C compilers.

An example of one such warning would be:

Unicode escape sequence is too long. Use \u{...} to disambiguate.
UnicodeIdentifier = [a-zA-Z_\u007F-\u10FFFF] [a-zA-Z0-9_\u007F-\uFFFF]*
                                         ^

Plus the corresponding JFlex Maven plugin option.

[lsf37]

That's a good idea.