Password for proxies from external

Hi there!

If searched a lot and everywhere i found the same information...

when i want to set a http-proxy for git, the configuration goes to ~/.gitconfig

```
[http]
     proxy = http://username:password@proxy.dings.bums:1234

```

So the proxy password is in cleartext written down to the config-file. The git own password-store can not be used... even no external like pass.

For aliases i can set something like
```
[alias]
     blafoo = "!f() {do something on shell}; f"
```

But this does not work for proxy-settings.

There should be a mechanism to read a password or the credentials from the shell in proxy-settings. Something like bash-subshell-construct.

```
[http]
     proxy = http://$(pass show key|grep login):$(pass show key|head 
-n1)@proxy.dings.bums:1234
```

or

```
[http]
     proxy = "!u() {pass show key|grep login}; !p() {pass show key|head 
-n1}; http://u:p@proxy.dings.bums:1234"
```

This would close the security-hole of cleartext-credentials in config-files...

thank you

Jakob