Add an ability to use "elif" in for statement and while statement besides "else"

Example
Now:
```python3
for i in range(j):
    ...
else:
    if i > 5:
        ...
    else:
        ...
```
Shall be:
```python3
for i in range(j):
    ...
elif i > 5:
    ...
else:
    ...
```

Changes to the language syntax usually require extensive discussion and a PEP.

To move forward, your next step would be to launch a discussion on the python-ideas newsgroup.  If the idea advances to an approved PEP, this tracker issue can be re-opened.