Update bumpversion to use prereleases

Author: asvetlov

.bumpversion.cfg

@@ -1,7 +1,17 @@
 [bumpversion]
-current_version = 0.14.1
+current_version = 0.14.2a0
+parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)((?P<release>[a-z]+)(?P<prerelease_number>\d+))?
+serialize =
+        {major}.{minor}.{patch}{release}{prelease_number}
+        {major}.{minor}.{patch}
+
 commit = True
 tag = True
 
 [bumpversion:file:aiohttp/__init__.py]
 
+[bumpversion:part:release]
+optional_value = b
+values =
+        a
+        b