Conditional proxy support

Our environment requires the use of a proxy for outbound internet access. Thus we need proxy support for alert destinations like PagerDuty.
Go does support the `http_proxy` and `https_proxy` environment variables, however these appear to control ALL http calls, which is not what we want as we do not want to use the proxy server for access to influxdb.

So we need some way of controlling what uses the proxy.
2 solutions I can think of:

1) Add a parameter to alert handlers which might need a proxy. E.G. `alert().pagerDuty.proxy('http://foo:bar@proxyhost:3128')`.

or

```
[pagerduty]
enabled = true
proxy = "http://foo:bar@proxyhost:3128"
```

2) Add a per-site proxy list to the config. E.G.

```
[proxy]
events.pagerduty.com = "http://foo:bar@proxyhost:3128"
```

^if toml is even capable of such a config