fix: duplicate case values

The '-t' flag was used twice: for embedding timecode and for setting
the title.

The title option was introduced earlier, so I've changed the timecode
option to use '-T'.
Pass filename to xml::write_data

Based on #7
Fix buffer overflow in graph timecode generation

char timecode[10] is too short for null-terminated string,
sprintf replaces the terminator, resulting in buffer overflow
or segmentation fault - especially with compiler optimization