Initial draft specification for age plugins (#5)

* Initial draft specification for age plugins

Migrated from https://hackmd.io/@str4d/age-plugin-spec so this is a
relatively complete draft.

* age-plugin: Define Base64 as the standard alphabet with no padding

* age-plugin: Add reference to v1 age header format

* age-plugin: Remove hackmd formatting header

* age-plugin: Tighten specification of allowed plugin names using ABNF