Custom time format

Add an extra parameter to allow the time format to be customised.
## Suggested format:
- `%m` is minutes so far, `%s` is seconds so far
- `%M` is minutes total, `%S` is seconds total
## Example specification:

```
time-format : "<span>%m:%s</span> / %M:%S"
```

would output:

```
<span>02:01</span> / 04:58
```
