Added possibility to use nested context objects

I needed this type of functionality e.g. [here](https://github.com/jojanaho/piechopper/blob/master/server/mgmt/install.sh), and ended up implementing it into my own [Gruntfile](https://github.com/jojanaho/piechopper/blob/master/Gruntfile.coffee#L7). Might be worthwhile addition into the library?

Shouldn't break the compatibility, except in really rare cases where someone has used '.' in their context variables, and used it only with @echo (since with @if it would break due to usage of `with`).

All tests pass, new ones included.