Support conditional template instructions

The form is either
```erlang
{'if', ConditionVariable :: atom(), InstructionsIfTrue :: list() | tuple()}.
```
or
```erlang
{'if', ConditionVariable :: atom(), InstructionsIfTrue :: list() | tuple(), InstructionsIfFalse :: list() | tuple()}.
```

@tuncer maybe the headline in basho/rebar should state the fact the repo moved a bit more aggressively (caps, exclamation marks), as self-referential links are common practice in repo descriptions I completely missed it.