Compose duties for public API

[stellarhoof]

Duties can be executed in different environments (non-exhaustive list):

• On (actual) CI
• On the local machine
  • With github metadata (danger pr and danger ci)
  • Without github metadata (danger local)
• (Internal) For testing purposes

Different duties have different implicit assumptions about their
environment right now, but those assumptions are not checked. Clients of
our library have to write those checks themselves if they want to have a
robust dangerfile that works in a variety of scenarios without breaking.
This is error prone for consumers of our API.

We can provide public helpers that check those assumptions and fail if
they aren't met. Users can compose them with duties they write
themselves if they want. We can also use them internally when exporting
our duties to the world. Ex:

src/duties/pr.js:

module.exports = {
  // No changes here. Export uncomposed duties for testing
}

src/duties/index.js:

let pr = require('./pr')
let autoFix = require('./autoFix')
let { checkGithub } = require('./utils')

module.exports = Object.assign(
  {},
  _.mapValues(duti => _.flow(checkGithub, duti), pr),
  _.mapValues(duti => _.flow(checkActualCI, checkGithub, duti), autoFix),
  ...
)