Feature/unprivilege user

[warrensbox]

New feature

1. Create a custom binary path. Ex: mkdir /Users/warrenveerasingam/bin (replace warrenveerasingam with your username)
2. Add the path to your PATH. Ex: export PATH=$PATH:/Users/warrenveerasingam/bin (add this to your bash profile or zsh profile)
3. Pass -b or --bin parameter with your custom path to install terraform. Ex: tfswitch -b /Users/warrenveerasingam/bin/terraform 0.10.8
4. Optionally, you can create a .tfswitch.toml file in your terraform directory.
5. Your .tfswitch.toml file should look like this:

bin = "/Users/warrenveerasingam/bin/terraform"
version = "0.11.3"