Improve .ini reads

[ryanblock]

Describe the problem underlying the enhancement request

Two main things going on with our .ini (~/.aws/credentials) support:

1. We rely on a relatively large (relatively speaking) dependency: ini, which is about ~20KB. I know, that's not bad, but the whole core codebase (without vendored code) is less than 100KB.
2. It's been a bit buggy when there are comments in the credentials file

Describe the solution you'd like (if any)

I think we should roll our own, probably based on what @mhart did here: https://github.com/mhart/awscred/blob/master/index.js#L320

Describe alternative solutions you've considered

That's kind of it!

Additional context or notes

No response