Bump npm version to 0.20.4.

Bump npm version to 0.20.3.

Bump npm version to 0.20.2.

Bump npm version to 0.20.1.

Bump npm version to 0.20.0.

The minor version bump is due to the extent of changes in PR benjamn#712, though
they should all be backwards compatible.

0.19.1

0.19.0

0.18.10

0.18.9

Bump npm version to 0.18.8 in preparation for publishing.