[ygrok] -m for multiline patterns

[preaction]

This will allow parsing, for example, iostat output, but it could cause a massive memory usage (if we fail to match, we will collect lines until we do match and can finally get rid of them, but if we continue to fail to match, we'll never flush our buffer).

We can (probably?) detect how many lines the pattern matches and only keep that many in the buffer. Alternatively, we can allow a "--buffer" size option.