Add support for custom formatters

[k-yle]

This PR adds support for custom formatters instead of vfile-reporter, like what eslint does.

Use case: you want machine-readable output to integrate with other CI tooling, such as the Gitlab CI .

[wooorm]

Hi there!

unified also allows custom formatters, and this is built into the code we’re using: see engine options the reporter and reporterOptions. I’m fine with adding this but don’t see why we shouldn’t use what’s build in already

[k-yle]

Hi @wooorm, it makes a lot more sense doing it that way! I've changed it to use vfile reporters like unified does, which works and the CI passes (2nd to last commit).

However, when I tried to use reporter and reporterOptions, I get the following error from unified which I can't figure out (latest commit):

Error: Cannot `stringify` without `Compiler`
  at assertCompiler (./node_modules/unified/index.js:444:11)
  at Function.stringify (./node_modules/unified/index.js:347:5)
  at stringify (./node_modules/unified-engine/lib/file-pipeline/stringify.js:45:23)
  at wrapped (./node_modules/trough/wrap.js:25:19)
  at next (./node_modules/trough/index.js:57:24)
  at Object.run (./node_modules/trough/index.js:31:10)
  at run (./node_modules/unified-engine/lib/file-pipeline/index.js:29:10)
  at wrapped (./node_modules/trough/wrap.js:25:19)
  at next (./node_modules/trough/index.js:57:24)
  at done (./node_modules/trough/wrap.js:55:16)

this is from the latest commit in this branch. any advice would be appreciated

thanks!

[wooorm]

Awesome, thanks!

The out removal is probably causing the error.

And some other tips inline!

[k-yle]

thanks for the help @wooorm, I've made the changes you suggested, and all tests are passing again

[wooorm]

thanks @k-yle, released!