Use short URL for png link

[ryanve]

use https://git.io/bae_png short URL made for #5

curl -i https://git.io -F "url=https://github.com/s9a/bae/issues/5" -F "code=bae_png"

HTTP/1.1 201 Created
Server: Cowboy
Connection: keep-alive
Date: Tue, 23 Oct 2018 12:07:08 GMT
Status: 201 Created
Content-Type: text/html;charset=utf-8
Location: https://git.io/bae_png
Content-Length: 35
X-Xss-Protection: 1; mode=block
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-Runtime: 0.094257
X-Node: baac46f9-21c6-4f02-aed1-29338b90777e
X-Revision: 392798d237fc1aa5cd55cada10d2945773e741a8
Strict-Transport-Security: max-age=31536000; includeSubDomains
Via: 1.1 vegur

https://github.com/s9a/bae/issues/5