Add Markdown formatter

[ksonney]

This adds a basic markdown output formatter. Based on the ascii art formatter, with the icons from the emoji formatter.

Sample output from go run main.go -u imperial -l 27312 -d 1

Weather for Pittsboro, US

scattered clouds ☁️ 95 (97) °F

Forecast for Wed Aug 03

Morning	Noon	Evening	Night
clear sky	scattered clouds	scattered clouds	scattered clouds
☀️ 80 (80) °F	☁️ 95 (97) °F	☁️ 95 (97) °F	☁️ 95 (97) °F