Extract plugins into separate repos

[decompil3d]

Right now, pullie plugins are in the core repo. To allow better extensibility, pullie should load plugins from external repos (e.g. pullie-plugin-jira).

[rxmarbles]

a bit late to this but we could potentially make pullie a monorepo and still allow pullie to handle external plugins similar to how gasket works. Thoughts @decompil3d ?

[decompil3d]

True -- could definitely explore that possibility