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/d3.py

@@ -0,0 +1,44 @@
+from lib import str_to_ints
+from collections import defaultdict
+
+
+def part_1(data):
+    claims = defaultdict(int)
+    for line in data.splitlines():
+        id, x, y, w, h = str_to_ints(line)
+        for dx in range(w):
+            for dy in range(h):
+                claims[(x + dx, y + dy)] += 1
+
+    r = sum([1 for v in claims.values() if v > 1])
+    print(r)
+
+
+def part_2(data):
+    claims = defaultdict(list)
+    all_ids = set()
+    for line in data.splitlines():
+        id, x, y, w, h = str_to_ints(line)
+        for dx in range(w):
+            for dy in range(h):
+                claims[(x + dx, y + dy)].append(id)
+        all_ids.add(id)
+
+    for xs in claims.values():
+        if len(xs) > 1:
+            for x in xs:
+                if x in all_ids:
+                    all_ids.remove(x)
+    assert len(all_ids) == 1
+    print(all_ids.pop())
+
+
+def main():
+    with open("i3.txt") as f:
+        data = f.read()
+    part_1(data)
+    part_2(data)
+
+
+if __name__ == "__main__":
+    main()