add argument support for `flamegraph`

# summary

this adds the extremely rudimentary ability to pass args from the `flamegraph` command line tool, to the `svg` outputter.  It works like:

```
$ flamegraph width=2500 title='Hello World' ...
```

this is related to https://github.com/davepacheco/node-stackvis/issues/10

# issues

this approach is nice because the user can specify arbitrary attributes that are passed as `args` to the backend function, so it won't be needed to be changed if the backend ever supports new options.  However, it's pretty dirty, the best approach would probably be to incorporate `posix-getopt` in this tool, and specify the options explicitly instead of using the approach in the PR.