AMP support

[fstanis]

I'm experimenting with using PostHTML and htmlnano with a page I've built using AMP and, while it works fine right now (as AMP is just HTML), I think there are some minor tweaks to htmlnano that would make it even more useful.

Notably:

• AMP has some additional attributes that could be added to collapseBooleanAttributes, such as amp, amp-custom, amp-boilerplate.
• mergeStyles and minifyCss should ignore the AMP boilerplate (<style amp-boilerplate>...</style>).
• amp-custom should be preserved on the merged <style> tags, if present (maybe just include this in styleKey).
• There may be additional attributes to be added to removeRedundantAttributes and removeEmptyAttributes (still looking into this).

I'm happy to send a PR with these changes, but wanted to start some discussion first. Would you be OK with accepting these additions to htmlnano?

[maltsev]

Would you be OK with accepting these additions to htmlnano?

Totally! I'll appreaciate it if you implement these minifications.

[fstanis]

#59 adds some of the things mentioned. I'll still need to look into whether additions can be made to removeRedundantAttributes and removeEmptyAttributes, but this should make htmlnano a lot more useful for AMP pages already.

[maltsev]

Great!

@fstanis are you going to submit new PRs this week (except #60)? If yes then I'll wait for your new PRs before releasing a new htmlnano version. Otherwise I'll release it as soon as #60 gets merged.

[fstanis]

I think I'll close this for now. It seems AMP support is in a good place now based on my testing - if we missed something, it's easy to reopen.