parser: accept triple quoted strings
nitc: handle triple-quoted strings

Signed-off-by: Jean Privat <jean@pryen.org>
nitg: handle triple-quoted strings

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add test_string_triple.nit

Signed-off-by: Jean Privat <jean@pryen.org>
Merge branch 'triple_quoted_strings'