Changing import_stackup to handle filename from argument

[oskarvh]

Hi,
First of all, awesome work, I'm currently using your project inside of another python script, so I'm not using it directly from the command line. But I was super excited when I found this repo!

Hence, I do want to have some flexibility when it comes to the placement of files. First snag I hit was the assumption that the stackup.json file is found under fab/stackup.json. This is generally not an issue but I've opened a PR to put this as a default argument instead of a constant in the function.

I couldn't find any more issues, but if I do I'll try to fix them and contribute back.

Link to PR: #7

[oskarvh]

On second look, there are a lot of paths that are assumed here, which is fine since the intention is to run this stand alone.
I might do a quick rethink of this PR, because I think there is a cleaner way of doing what I want, that involves overloading the BASE_DIR instead.

I'm liking this project, and I don't think that there is that much missing from converting this to an API.

[oskarvh]

I've created a PR into my own fork of this repo: oskarvh#1 in order to properly test it before creating a PR into this repo.

But this seems to be in line with what I'm looking for.
The general idea is to move the default working directories from constants into the config class such that it's possible to override them.