don't assume numeric line numbers

`PHP_CodeSniffer_Reports_Full` assumes that line numbers in the report are numeric. This isn't always the case. We use this class to display custom reports and that yield line ranges, such as "112 - 118".

So, if a report has two messages, for lines "156 - 172" and "27", the width for the line column is determined as 2, because "27" > "156 - 172".

This PR calculates the actual maximum string length.