Add login shell arguments as a preference (default --login)

Fixes profile/bashrc etc. from *not* being parsed when entering an aws-creds shell. Example:

I use the following configuration for my shell's prompt:
```
local GIT_INFO='$(__git_ps1 " (%s)")'
```
When entering a new aws-creds shell, I always see:
```bash
$ aws-creds shell test
bash: __git_ps1: command not found
(aws test) $
 ```

Until I source my `.bash_profile`. Adding the `--login` option by default will force profile to be sourced whenever entering a new shell. This has the added benefit of allowing rbenv/rvm to work immediately inside the shell as well.

Changes:
* Adds `Shell arguments (default: '--login'):` step to `aws-creds init`
* Shell args are passed to the shell on instantiation `aws-creds shell <name>`