Add method to get access to a file's source in SimpleReportingFiles

[Boscop]

After I have gotten the byte index with let str_start = files.byte_index(file_idx, line_nr, 0).unwrap(); I need to know the length of that line, so that I can use it as span length.
Please add a way to get access to a file stored in SimpleReportingFiles, so that I can do
let line_len = files.file(file_idx)[str_start..].position(|c| c == '\n');
to get the line len.