Added git functions

Hey, this POC of how to call `git_describe_tag` as a function so you don't need `source.gut_url` in your meta.yaml

This needs to be cleaned up but, I tested this on my mac with the following meta.yaml

```yaml
package:
  name: conda_test_package
  version: {{git_describe_tag(RECIPE_DIR)}}

build:
  number: 1

```

cc @tswicegood  @asmeurer