creates the .aws home directory if it doesn't exist

[morganda]

I updated the initial setup so that it would create the $HOME/.aws directory as well as the credentials file if it doesn't exist to make initial setup a bit smoother. I can create tests for this if desired, but I didn't see any others so I passed for now. That said it worked fine locally - below you can see the output.

[morgana@Morgans-MacBook-Pro:~/src/aws-mfa]
>>>aws-mfa - master
$>rm -rf ~/.aws

[morgana@Morgans-MacBook-Pro:~/src/aws-mfa]
>>>aws-mfa - master
$>aws-mfa --setup
Could not locate credentials file at /Users/morgana/.aws/credentials, would you like to create one? [y/n]y
Profile name to [default]:
...

[morgana@Morgans-MacBook-Pro:~/src/aws-mfa]
>>>aws-mfa - create_home_aws_dir_if_not_exists
$>ls -al ~/.aws/credentials
-rw-r--r--  1 morgana  staff  0 Mar  8 11:27 /Users/morgana/.aws/credentials

Additionally, I used f-strings, but I'm not sure if we want to maintain python2 compatibility. Thoughts?