Environment variable in .cookiecutterrc file evaluation

* Cookiecutter version: 1.5.1
* Python version: 3.6
* Operating System: Windows

Hey there! First of all thank you for this tool, I've always struggled to find something flexible and customizable as powerfull like this! :)

As title goes, I'd like to use the `.cookiecutterrc` with couple of custom aliases. In these aliases I use custom paths with some environment variables like this:
```
abbreviations:
mt:%MYVAR%/path/to/my_template
```

Running `$ Cookiecutter mt` this is the error Cc spits:

> A valid repository for "%MYVAR%/path/to/my_template" could not be found in the following locations:
> %MYVAR%/path/to/my_template
> C:\Users\Gruber\\.cookiecutters/%MYVAR%/path/to/my_template

What can I do to make this work? Is it possible to use the env variables in aliases paths?

Thank you!

PS.
BTW why the doube 'rr' in the config file name (.cookiecutte**rr**c)? Got me quite a headache to get it read by Cc, eventually I noticed I was missing an r! :P )

Start with the `.cookiecutterrc` alias configuration and the `Cookiecutter mt` command using the Windows `%MYVAR%` example. Trace how the alias path is resolved, then verify that the expected expanded path is used while existing repository lookup behavior remains intact.

Written by the indexing model from the issue text.