escape modloadmsg

when you use something like this in a easyconfig, you need to scape $

```
modloadmsg = """To list all the available utilities execute ls \\$EBROOTPICARD"""
```

With this fix easybuild will take care of scaping 

actually, I'm wrong, `$` doesn't need to be escaped in Lua modules (I was confused since Lmod does require escaping when consuming Tcl modules)

so @pescobar: please move `CHARS_TO_ESCAPE` into the `ModuleGenerator` class, then it can be handled accordingly in #1060