Batch update

[willmcgugan]

Fixes #1807

Adds "batch updates" to app and Markdown improvements

The basic concept of batch updates is that you can defer repaint / layout while updating the screen. The reasons are two-fold:

1. It does less work by replacing multiple operations with one.
2. It reduces flicker because the screen doesn't update until you are finished.

In Markdown I've improved handling of lists, soft breaks, and added horizontal rules.