Improve semver-adherence of library

[passcod]

Currently this project doesn't consider the library to impact semver, so breaking changes to the library interface don't bump semver. While I'm thinking of changing this policy later on, in the meantime steps can be taken to improve version-to-version compatibility and avoid ecosystem breakage as usage increases.

• Document semver policy in the API docs
• Handler trait definition makes it difficult to define third-party handlers #133 / #133: leave Handler initialization to watch caller #134: allow handlers to be initialised by caller
• Use a builder for Args