Handle bash init scripts better

[wbyoung]

We need to better handle .bashrc vs .bash_profile on different platforms.

See the conversation on #30.

[ljharb]

See https://github.com/creationix/nvm/labels/installing%3A%20profile%20detection as well. Profile detection is insanely difficult.

[wbyoung]

@ljharb thanks for the heads up.

Maybe the best approach would be to make it interactive if more than one option exists.

[johntron]

Here's an idea: nvm-sh/nvm#1042

[ttaira]

Ubuntu would like .bashrc. It doesn't source .bash_profile or .profile by default, so avn is just not enabled on any environment.

[wbyoung]

@johntron @ttaira happy to review any PR that addresses this. It certainly would make things easier on everyone!