Add ability to auto-capture har files

TE-449 
Small style nitpick: I find the following to be a little easier to read:

```
msg = (
    "Browser does not support screenshots.  "
    "Could not save screenshot '{name}'"
).format(name=name)
```

Also, it looks like `name` should be a kwarg in the format string (otherwise it'll give an error).  Right?