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/d2.py

@@ -0,0 +1,53 @@
+from lib import LETTERS_LOWER
+
+
+def part_1(data):
+    two_count, three_count = 0, 0
+    for line in data.splitlines():
+        count = [line.count(c) for c in LETTERS_LOWER if line.count(c) > 1]
+        if 2 in count:
+            two_count += 1
+        if 3 in count:
+            three_count += 1
+    r = two_count * three_count
+    print(r)
+
+
+def equal(a, b):
+    c = 0
+    if len(a) != len(b):
+        return False
+    for i in range(len(a)):
+        if a[i] != b[i]:
+            c += 1
+    if c == 1:
+        return True
+    return False
+
+
+def strip(a, b):
+    r = ""
+    for a, b in zip(a, b):
+        if a == b:
+            r += a
+    return r
+
+
+def part_2(data):
+    lines = list(data.splitlines())
+    for i in range(len(lines)):
+        for j in range(i + 1, len(lines)):
+            a, b = lines[i], lines[j]
+            if equal(a, b):
+                print(strip(a, b))
+
+
+def main():
+    with open("i2.txt") as f:
+        data = f.read()
+    part_1(data)
+    part_2(data)
+
+
+if __name__ == "__main__":
+    main()