CSS tweaks to improve display of empty paragraphs & tables

Two minor tweaks to make the display of the caret within a table more professional looking:

1. Use ``::after`` pseudo-element to prevent empty paragraphs from collapsing rather than min-height
2. Horizontally centre caret slightly better between characters

![adjusted-layouts](https://cloud.githubusercontent.com/assets/1052079/3951536/200b6eea-26d8-11e4-90f1-2f7424eb4ec9.png)