opIndex, opDollar and opSlice override on ParseTree

[Zardoz89]

Override opIndex, opDoller and OpSlice like is suggested in #258

So p.children[x].children[y] could be p[x][y] or p.children[1..$-1] could be p[1..$-1] , etc.

[veelo]

Thanks! Would you mind documenting this in the wiki? No hurry, this becomes only relevant when we tag a new release I guess.

[Zardoz89]

I would take a look, but my english it's horrible. Any idea when the next version would be ready ? (also I noticed that the unit tests are actually failing on the master branch)

[veelo]

If you want, I can brush off language mistakes. Otherwise I can take over the documentation.
I could tag a new release anytime, were it not for #272. I hope that situation can be improved.