Paste improvements

[bantic]

This improves the ergonomics of pasting into content-kit. Pasting a single non-markup-section (like a card or a list section) now works properly (previously it would be ignored), and it replaces the empty section if the cursor is in an empty section.

Adds validations around tagNames that helps prevent corrupting mobiledocs by creating markupSections with, e.g., "li" tagNames.

It is still possible to confuse postEdtior#insertPost when inserting a post that contains a listSection when the cursor position is already in a listItem, but due to the tagName validations the mobiledoc does not end up with incorrect markup sections in it as a result.

• handle pasting an empty post
• Handle passing a non-markup section (list or card) to insertPost
• make insertPost replace empty sections when pasting
• Restrict tagNames to valid ones for markup section, list section, list item

fixes #196
fixes #190