Feature/map attributes

[Daylon]

Following up issue #8 this PR proposes to add further rule-to-attribute capability by letting the user to add custom maps to any HTML element.

Use case: dedicated email markups à la MJML try to remove some complexity to improve semantics readability; they're doing so by using custom HTML element which, once compiled, are converted to one or more nested elements optimized to be Email HTML compliant. However, such engines tend to force-remove style attributes.

Inline-CSS may help keep existing pipelines clean (Gulp, etc.) by using its capabilities to keep custom style rules separated from markup, only applying properly declared attribute maps; especially since the said markup may combine both regular HTML.classname and custom markup —MJML, etc.

Example:

var options = {
            removeStyleTags: true,
            applyAttributesTo: {
              'mj-section': [
                'full-width',
                'text-align',
                'background-color'
              ],
              'mj-section,mj-text': [
                'padding'
              ]
            }
        };

[jonkemp]

Please fix lint issues first.

[coveralls]

Coverage increased (+0.2%) to 98.684% when pulling 09ede30 on Daylon:feature/map-attributes into be4cbf9 on jonkemp:master.

[coveralls]

Coverage increased (+0.06%) to 98.571% when pulling dd962bf on Daylon:feature/map-attributes into be4cbf9 on jonkemp:master.

[coveralls]

Coverage increased (+0.06%) to 98.571% when pulling f3a52f4 on Daylon:feature/map-attributes into be4cbf9 on jonkemp:master.

[Daylon]

Here is it. Sorry about the delay. To elaborate a bit further on this proposal: it's really about being able to use any exotic markup language and still keeping markup and stylesheets separated in the source files when creating email HTML… Which, imho, fits well with this project.

[jonkemp]

The tests are good but I would like to see a better explanation of this along with an example in the readme.

[Daylon]

Hi @jonkemp, I didn't have time to take care of this earlier but if you're still interested, I opened up a new repo where I do use this fork to build a series of emails. Hope this helps enough to see if it's revelant or if you're better off without.