Support PEP 440 version scheme

Allow specifying x.y.z.dev0

Now when we bump a minor it will automatically bump to dev.

current_version = 0.2.1
$ bumpversion patch
new_version = 0.2.2.dev0

If we want to release from any dev version:

$ bumpversion dev=4

setup.cfg

@@ -2,6 +2,10 @@
 current_version = 0.2.1
 commit = True
 tag = True
+parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)(?:\.dev(?P<dev>\d+))?
+serialize = 
+	{major}.{minor}.{patch}.dev{dev}
+	{major}.{minor}.{patch}
 
 [bumpversion:file:setup.py]
 search = version='{current_version}'
@@ -11,6 +15,15 @@ replace = version='{new_version}'
 search = __version__ = '{current_version}'
 replace = __version__ = '{new_version}'
 
+[bumpversion:part:dev]
+values = 
+	0
+	1
+	2
+	3
+	4
+optional_value = 4
+
 [bdist_wheel]
 universal = 1