Allow to unbuffer csv output

[msimonin]

Add a flag to the command line: -u, --unbuffered

When set the output file (if any) will disable the default python buffering and the output will be flushed after every single line (line buffering mode).

This pach offers the opportunity to

• get a consistent view of the metrics when tailing the csv file
• get the metrics regardless the process termination cause (currently SIGINT will flush the data but not SIGHUP nor SIGTERM for instance)

ISSUE TYPE

• Feature pull-request

DSTAT VERSION

Dool 0.9.10 (master version)

SUMMARY

see commit message