Support output to files directly instead of just via output redirection

[jasonrhansen]

Closes #193

Adds --outfile option to all of the CLI programs so you can output directly to a file instead of STDOUT.

[jonhoo]

Hmm, what's the benefit of this over just having the caller do output redirection?

[jasonrhansen]

Based on what @saethlin reported in #193, it looks like it was requested for performance reasons because "the standard library's stdout wrapper is trying to detect newlines and flush on them".

[jonhoo]

Ah! My proposal then is rather that we be smarter about how we access stdout. The standard trick here is to detect if STDOUT is a TTY, and if it is, then line-buffer, otherwise use a BufWriter. This PR is probably a good starting point for that, since we'll then have to write to a writer directly anyway. That means we can continue to use the primitives the OS provides us with, while still supporting both interactive and batch use-cases.

[jasonrhansen]

So do you mean something like this?

    if atty::is(atty::Stream::Stdout) {
        flamegraph::from_files(&mut options, &infiles, io::stdout().lock())?;
    } else {
        flamegraph::from_files(&mut options, &infiles, io::BufWriter::new(io::stdout().lock()))?;
    }

[jasonrhansen]

Opened #206 as a better solution.