fix: Deprecate istanbul-api.

[coreyfarrell]

BREAKING CHANGE: The istanbul-api module is now deprecated. Any modules
which use istanbul-api should migrate.

Fixes #321

[coreyfarrell]

The plan is that after merging this we will perform a semver-major release of istanbul-api (3.0.0) to npm, then immediately deprecate that version. Once that is completed a follow-up will delete packages/istanbul-api and remove the reference to it from greenkeeper.json and .eslintignore.

Users of packages which depend on istanbul-api@2 will not get any warning, maintainers will get the deprecation notice if they try to upgrade.

[lukeapage]

IMO the reporter part of api should be moved to istanbul-lib-report to provide a easy way to do reporting that gives us the flexibility to change. It should probably just be the interface to that module and then not reference the reports - as it is, to use these 2 modules you have to know quite a few very specific implementation details and it ties istanbul up to breaking changes if any implementation details change or new reports come in that aren't strict visitors (like html-spa)

[coreyfarrell]

@lukeapage I appreciate your concerns and I'm absolutely open to suggestions to improve the istanbul-lib-report interface (sorry for the slow response past couple weeks have been difficult for me). As mentioned on the Modern HTML PR I'm open to semver-major actions to istanbul-lib-report / istanbul-reports which would shift some of the API burden away from the code running reports.

Just so you are aware part of the problem is a lack of knowledge about istanbul-api among maintainers. It would help me to know of specific istanbul-api features which you would like to see supported by istanbul-lib-report. Sorry if this sounds like a silly question but I've never used istanbul-api so I can't understand the value of it.

[lukeapage]

I’ve only ever used the api lib to make reporters straight forward so my concerns are satisfied. I could have a go at a new api proposal if I get some time.

[coreyfarrell]

@lukeapage please hold off for now, chances are high that I will have a PR posted this weekend to improve istanbul-lib-report (I already have some WIP). I'll tag you on that PR when it's created.

[lukeapage]

Sounds good!

[bcoe]

\o/ I'm excited about this; remove a layer of indirection.