Summarize: calculate stats as-we-go

Instead of loading the whole file into memory, calculate everything one
line at a time.

Most of these formulas are accurate enough with the kind of data we're
talking about. They can get off when dealing with large discrepancies in
magnitude.
Summarize: add diff to ratio output
Summarize: allow STDIN to replace -f

Cause sometimes you wanna pipe it.
Merge summarize-refactor-for-speed (#4) into master

Summarize: calculate stats as-we-go

Instead of loading the whole file into memory, calculate everything one
line at a time.

Most of these formulas are accurate enough with the kind of data we're
talking about. They can get off when dealing with large discrepancies in
magnitude.

Now we can process arbitrarily large files without running out of
memory.