Width of tables not working

[ian-howell]

Describe the bug
The width of markdown tables is currently fixed at around 78 characters, making them borderline unusable.

To Reproduce
Steps to reproduce the behavior:

1. Create the following markdown file:

# Terminology
| Term | Definition |
| --- | --- |
| target | The file that is generated by a rule (with the exception of PHONY targets) |
| dependency | A file (or target) that is used to generate a target |

2. Render it with slides.
3. Observe that the table is restricted to a narrow margin

Expected behavior
The table should use as much horizontal space as needed before word-wrapping.

Screenshots