Added filter marked:use for direct access to the marked.use function

[crycode-de]

Background:
The marked-alert extension enables usage of GFM alerts.
To use this extension (and maybe some others) the current marked:extensions filter is not enough since it only provides the extensions property but the altert extension also requires walkTokens to be set in marked.use(...).

This PR adds a marked:use filter allowing the usage of the marked.use function in a theme.

Usage example:

const markedAlert = require('marked-alert');

hexo.extend.filter.register('marked:use', function (markedUse) {
  markedUse(markedAlert());
});

[yoshinorin]

LGTM 👍

Thanks :)

[coveralls]

coverage: 100.0%. remained the same
when pulling d192435 on crycode-de:master
into 1b09354 on hexojs:master.

[yoshinorin]

@crycode-de
We released a new version (v6.3.0) which includes this feature.
https://www.npmjs.com/package/hexo-renderer-marked/v/6.3.0

Thank you :)