define alias for the modulefile in your easyconfigs

first working version. It needs more testing

with this you can add this to your easyconfigs

```
modextraalias = {
    'gatk':               'java -jar $root/gatk.jar',
}
```
 and the alias will be added in the generated modulefile

`set-alias   gatk        "java -jar $root/gatk.jar"`
hmm, maybe `modextra_aliases` is better? plural, but also avoid the double `a` to make it more readable (although that doesn't match the `modextrafoo` pattern that was being used for the others