Markdown rendering on the terminal is limited by performance

[jessitron]

Investigating #76 , we noticed that rendering a list of 16 commands never finished.
from testing:
// 3s for 12 (at 943 characters)
// 10s for 13
// 50s for 14

This is a performance limitation. We upgraded to marked@0.5.0 but the problem is still there.

Our workaround is to skip markdown rendering for messages longer than 1000 characters / 12 lines.

A better-performing parser would be better.