Decouple linkify in separate plugin

[TrySound]

Closes #343

linkify depends on autolinker which is quite heavy. The option is
disabled by default and not used most of the time.

https://bundlephobia.com/result?p=autolinker@3.1.0

In this diff I decoupled linkify into plugin which can be accessed like
this

import Remarkable from 'remarkable';
import linkify from 'remarkable/linkify';

const md = new Remarkable({ html: true })
  .use(linkify);

For UMD build linkify provided out of the box

const md = new window.Remarkable({ html: true })
  .use(window.Remarkable.linkify);

[coveralls]

Coverage increased (+0.0007%) to 98.584% when pulling d887353 on decouple-autolinker into 19c515a on master.

[coveralls]

Coverage increased (+0.0007%) to 98.584% when pulling d887353 on decouple-autolinker into 19c515a on master.

[coveralls]

Coverage increased (+0.002%) to 98.586% when pulling 766f50a on decouple-autolinker into 19c515a on master.

[TrySound]

@jonschlinkert @doowb @shockey are you okay with this change?

[doowb]

I like the change, it looks great!

I'm assuming the reason for doing it this way is because it gives bundlers a chance to tree shake and exclude autolinker from the bundle when linkify is not used.

I think this change should wait until a major version bump is ready. Even though the default for linkify is false, any users that have linkify set to true will have different behavior and will need to upgrade.

[TrySound]

I can add warning that linkify option is replaced with plugin. Major bump is almost ready (at least from my POV). This PR is a blocker for #350.

[doowb]

In #350 you mentioned: internals are hidden and changing them will not affect users. I assumed that once the changes are made and the code is bundled that version 1.7.2 could be published . Then 2.0.0 could be published with this PR (and any others that might have a breaking change).

If you're saying that all of the other stuff would require a major bump, then I'm fine with this PR being merged now. Especially because I'm excited to see all the work you've been doing get released 😀 , but I'm still wary about breaking changes in a patch bump.

If you have a plan for the releases, I'm good with that and I'll approve my review.

[TrySound]

I think removed bower support is a breaking change so we cannot publish 1.7.2.

[shockey]

Looks good to me as a major version bump