Add setSimpleTag() for basic custom tags (#511)

Still need to add some new docs for this, but this is what I was thinking for the implementation. Let me know if anything is missing or looks wrong. Thanks!
This isn't really going to work. You'll have to use the default variable parser here, since macros and loops duplicate and set new contexts for variable lookups.

Example:

```
{% for taco in sixpack %}
  {% my_tag taco %}
{% end %}
```

This would just end up giving you nothing, since the variable `taco` isn't defined on the global `options.local.taco`, but copied into compiled code that is somewhere like `_ctx.taco`.