Add exporting project summaries

This allows users to export a plain-text summary of a project which includes the variables, lists, scripts, costume names, and sound names of every sprite. It looks like this: https://gist.github.com/nathan/98a73b2d0824f1bbf81b#file-spirals-txt (the scripts are in [scratchblocks2](http://blob8108.github.io/scratchblocks2/) format)

Fixes #15.