Add PowerShell support for aliases

The current alias file syntax supports only bash syntax and is not compatible with Windows. For example, the example provided at https://github.com/awslabs/awscli-aliases define the function 'f' in bash compatible notation. See the below example for the alias 'myip' which would be stored in ~/.aws/cli/alias:
```bash
myip =
!f() {
dig +short myip.opendns.com @resolver1.opendns.com
}; f
```
When executed on a Windows host the following error is seen:
```bash
$ aws myip
'f' is not recognized as an internal or external command,
operable program or batch file.
```
A suggested resolution is in the pull request at https://github.com/aws/aws-cli/pull/4284 where PowerShell is used for the alias external command.