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.

2018/d1.py

@@ -0,0 +1,28 @@
+def part_1(data):
+    r = 0
+    for x in data.splitlines():
+        r += int(x)
+    print(r)
+
+
+def part_2(data):
+    seen = set()
+    r = 0
+    while True:
+        for x in data.splitlines():
+            r += int(x)
+            if r in seen:
+                print(r)
+                return
+            seen.add(r)
+
+
+def main():
+    with open("i1.txt") as f:
+        data = f.read()
+    part_1(data)
+    part_2(data)
+
+
+if __name__ == "__main__":
+    main()