flowtrace-extract: Tool for extracting particular data

[jonnor]

Should give machine-readable data, to compliment flowtrace-show (which gives human-friendly output).

Should reuse the filtering options (#10 #11 #12). By default should probably create a new Flowtrace file with "subtrace". But should also have other options to extract other things, for working onwards using other tools.

• Extract a particular graph
• Extract only data on edges
• (for completeness) Extract only groups

Maybe option for