Add a “common customizer” that handles common tags.

[timonus]

Things like b, i, u, and URLs.

[timonus]

Started here https://github.com/timonus/NSAttributedStringMarkup/tree/common-customizer.

[timonus]

Also em for i, strong for b, s/del for strikethrough, maybe others.

[coreform]

Here's a good target to hit, a list of HTML tags supported by Android TextView: https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/text/Html.java#784