Support for standalone flamegraph svg files

[ShaneHarvey]

The flamegraph support added in #166 is great, however it's difficult to share this information with others (in emails, bug trackers, etc..). It would be nice if there was a -flamegraph command line option to output a flamegraph svg file. Something like: go tool pprof -flamegraph ./binary ./profile.pprof

To accomplish this right now, one has to:

1. Install go-torch: go get github.com/uber/go-torch
2. Install brendangregg/FlameGraph
3. Finally, generate the svg: go-torch -b ./profile.pprof --binaryname ./binary. (make sure FlameGraph is on your PATH)

[ShaneHarvey]

Closing as a duplicate of #401.