Follow PEP8 in module naming

I just tripped up like this:
```
In [4]: iminuit.ConsoleFrontend()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-e8c1f7bebd8e> in <module>()
----> 1 iminuit.ConsoleFrontend()

TypeError: 'module' object is not callable
```
I propose to follow PEP8 and use `snake_case` for module names, i.e. to do the following renames:
* `iminuit/ConsoleFrontend.py` -> `iminuit/console_frontend.py`
* `iminuit/HtmlFrontend.py` -> `iminuit/html_frontend.py`

@piti118 Would it be OK if I do this?
Or would `iminuit/frontends/console.py` and `iminuit/frontends/html.py` be even nicer?