feat(power-assert): provide API for customization
feat(power-assert): customized assert should also be customizable
chore(power-assert): rebuild web bundle
docs(README): add docs about customization API
feat(power-assert): seeking for better names. `assertion` and `output` would be better.
chore(power-assert): rebuild web bundle
chore(package): update empower, power-assert-formatter and other devDependencies
docs(README): update README
chore(power-assert): rebuild web bundle
Merge pull request #14 from twada/customizable

Provide API for customization