Start solving 2018 problems

I have also updated get.py to download the problems as
`d<day>.txt` instead of `i<day>.txt`. That allows me
to get the day input via `__input__.replace('.py', '.txt')`
which is a little more concise. I don't know why
I didn't do this earlier.

README.md

@@ -86,3 +86,11 @@ written in Python.
 - Day 23: Multiple days... but super fun.
 - Day 24: 15:45 (48th)
 - Day 25: 41:00
+
+# 2018
+
+- Day 1: 1:49
+- Day 2: 10:53
+- Day 3: 6:16
+- Day 4: 25:16
+- Day 5: