Support bg differential flamegraph

http://www.brendangregg.com/blog/2014-11-09/differential-flame-graphs.html

Format example:

```
deflate::matching::get_match_length;core::iter::traits::iterator::Iterator::count 1 0
```

Start by reading the linked differential flame-graphs article and comparing its requirements with the provided folded-stack format example. Done means speedscope can accept and visualize bg differential flamegraph data; the issue names no files or tests, so identify the relevant import and rendering entry points before implementation.

Written by the indexing model from the issue text.