Support for multiple accounts

[Oppen]

I have two different Trello accounts, one provided by my employers for work and a personal one. Sadly, this client doesn't provide out-of-the-box support for this use case.

I'm now working in implementing this, but I'd like early feedback, thus the issue. I thought of a few ways to support this:

• Don't explicitly support it, but redirect the $HOME env var on launch so I can have two different config paths transparently. This is hacky but works with minimal effort.
• Have a flag to set the configuration dir. It requires minimal modifications. This is my current approach.
• Have support for multiple accounts in the main config file. This may require more changes, but it would allow for runtime switch between boards of many accounts, and also reduces the proliferation of folders of the previous two approaches.

Any preferences? Any other ideas? I don't want to give an estimate because I'm distracted and systematically procrastinating lately, but I think I should be able to have a PR soon.

[geolessel]

@Oppen I love the idea of having multiple accounts. I think the easiest, quickest to implement, and easiest to document would be to allow a flag that sets the configuration directory. I look forward to your PR.