Add configurable storage directory that defaults to `node_modules/.ca…

[mgcrea]

Checklist

• [X] tests and code linting passes
• [X] a test is included
• [X] the commit message follows commit guidelines

  Affected core subsystem(s)

• install
• build

  Description of change

Added a new config variable: env.IED_STORAGE_DIR

That defaults to node_modules/.cas.

Basically this enables us to address #96 and get a very clean folder layout (way friendlier for web-based IDEs!).

I like it as it really gives a cleaner result after an install, really shines vs. npm@3. So I think we could use a subfolder by default (.cas). However if you'd rather keep things the way they are, I'll gladly revert the default (while keeping it configurable by an environment var).

[alexanderGugel]

I think this is a great idea going forward. Node itself has node_modules hardcoded all over the place. There is no reason why we shouldn't keep it modular (and possibly support systems other than Node later down the road).

[mgcrea]

@alexanderGugel Is there anything blocking the merge? I can rename .cas to something else (like .ied), or keep the current behavior for now if you want.

[alexanderGugel]

Merged. Sorry for the delay.

I think .cas as a directory name makes sense... it's not a valid package name and hiding it is definitely a good idea :+1: