WIP: add svg backend to report.py

cc @mainakjas @agramfort 
no, don't use PIL directly either. Simply do:

```py
with open(temp_fname, 'rb') as fid:
    img = base64.b64encode(fid.read()).decode('ascii')
```
instead of 

```py
from scipy.misc import imread
img = imread(temp_fname)
```
that way we can avoid PIL