All plotting functions now accepts ax=None

Currently, some plotting functions raise an error when `ax=None` passed.

```
kmf.plot(ax=None)
AttributeError: 'NoneType' object has no attribute '_get_lines'
```

It is nice to accept `ax=None`, especially when plotting multiple lines on a single axes.

```
ax = None
# assuming kmfs is a list of KaplanMeierFitter instances
for kmf in kmfs:
    ax = kmf.plot(ax=ax)
```

`kwargs` needs an `ax