Add Memo-Runner chaining/composition support

[mdittmer]

A series of *Runner classes encapsulate the Memo composition to "run a task" from the command-line (see main/memo.es6.js for Runner entry points). Several of these tasks could be composed the same way memos are (with some massaging of the data, of course).

E.g., parse-blink and scrape-blink-linked, then process-idl (blink), then process-idl-from-reference (blink-linked with blink as reference), then copy outputs into data/idl/blink(/linked). This has been tested out with an ugly shell script wrapper, but it would be nice to be able to do it all in one go.