Added addErrorOnLine and addWarningOnLine

Specifying line number instead of $stackPtr when adding errors
enables to write custom sniffs using [nikic/php-parser](https://github.com/nikic/PHP-Parser).

Using PHP Parser greatly simplifies writing sniffs when there's no need to inspect whitespaces. For example when I want to check if use clausules at the beginning of the file are alphabetically ordered or whether all exceptions are used via fully qualified name.

Even if we don't consider writing sniffs with the help of PHP Parser, users can benefit from a more flexible and transparent API (in case they want to mark an error, don't have access to $stackPtr, but know about the line number).