Clarify expression alias logic

Instead of using tuple indexing use a namedtuple mapped from the
expression aliases. This makes readability / understanding of this logic
a little easier.

src/croniter/croniter.py

@@ -3,6 +3,7 @@
 
 from __future__ import absolute_import, print_function, division
 
+from collections import namedtuple
 import traceback as _traceback
 import copy
 import math
@@ -83,6 +84,20 @@ def is_32bit():
 for i in M_ALPHAS, DOW_ALPHAS:
     ALPHAS.update(i)
 del i
+
+ExpressionAlias = namedtuple('ExpressionAlias', 'regular, hashed')
+
+# Non-standard expression aliases
+EXPR_ALIASES = {
+    '@midnight': ExpressionAlias('0 0 * * *', 'h h(0-2) * * * h'),
+    '@hourly': ExpressionAlias('0 * * * *', 'h * * * * h'),
+    '@daily': ExpressionAlias('0 0 * * *', 'h h * * * h'),
+    '@weekly': ExpressionAlias('0 0 * * 0', 'h h * * h h'),
+    '@monthly': ExpressionAlias('0 0 1 * *', 'h h h * * h'),
+    '@yearly': ExpressionAlias('0 0 1 1 *', 'h h h h * h'),
+    '@annually': ExpressionAlias('0 0 1 1 *', 'h h h h * h'),
+}
+
 step_search_re = re.compile(r'^([^-]+)-([^-/]+)(/(\d+))?$')
 only_int_re = re.compile(r'^\d+$')
 
@@ -811,22 +826,14 @@ def _expand(cls, expr_format, hash_id=None, second_at_beginning=False, from_time
         # Split the expression in components, and normalize L -> l, MON -> mon,
         # etc. Keep expr_format untouched so we can use it in the exception
         # messages.
-        expr_aliases = {
-            '@midnight': ('0 0 * * *', 'h h(0-2) * * * h'),
-            '@hourly': ('0 * * * *', 'h * * * * h'),
-            '@daily': ('0 0 * * *', 'h h * * * h'),
-            '@weekly': ('0 0 * * 0', 'h h * * h h'),
-            '@monthly': ('0 0 1 * *', 'h h h * * h'),
-            '@yearly': ('0 0 1 1 *', 'h h h h * h'),
-            '@annually': ('0 0 1 1 *', 'h h h h * h'),
-        }
-
         efl = expr_format.lower()
-        hash_id_expr = hash_id is not None and 1 or 0
-        try:
-            efl = expr_aliases[efl][hash_id_expr]
-        except KeyError:
-            pass
+
+        # Expand expression aliases like `@midnight`
+        if efl in EXPR_ALIASES:
+            if hash_id is None:
+                efl = EXPR_ALIASES[efl].regular
+            else:
+                efl = EXPR_ALIASES[efl].hashed
 
         expressions = efl.split()