soak assign

Proposed at http://groups.google.com/group/coffeescript/msg/4a596472dda031ed:

```
# a.b = c if c?
a.b? = c

# [time, hour, minute] = that if /^(\d\d):(\d\d)$/exec input
[time, hour, minute]? = /^(\d\d):(\d\d)$/exec input

# (options) -> {what, how} = options if options?
({what, how}?) ->
```
