Macros execute commented out SQL, in my case creating a circular reference

Describe the bug

A clear and concise description of what the bug is. What command did you run? What happened?

Steps To Reproduce

Create a macro with the following code, and try to build any model

-- To use, call {{ my_macro() }}
{% macro my_macro() %}
	'order by 1'
{% endmacro %}

Expected behavior

The first line to be ignored because it’s an SQL file, so -- should be ignored

Screenshots and log output

maximum recursion depth exceeded while calling a Python object```

### System information
**Which database are you using dbt with?**
Redshift

**The output of `dbt --version`:**

installed version: 0.13.0 latest version: 0.14.2

Your version of dbt is out of date! You can find instructions for upgrading here: https://docs.getdbt.com/docs/installation

**The operating system you're using:**
Mac OSX 10.14.6

**The output of `python --version`:**
Python 3.6.8

### Additional context
Add any other context about the problem here.

[jonniepeller]

Thanks @drewbanin 😃

[drewbanin]

ah! Sure - that makes sense. Check out this issue – I think a good course of action would be to let users configure the file extensions for these types of resources. Strictly speaking, dbt models aren’t really SQL files either – it’s definitely reasonable to want to name them .jinja.sql, or similar!