Support Vault authentication on HTTPS datasource

[hairyhenderson]

It'd be cool to use the https datasource to grab secrets from HashiCorp's Vault.

A bunch of auth backends exist, but I'd like at least the following to work:

• Username & Password - Adding support for userpass vault auth backend #104
• AppID - Support for Vault datasources (app-id & token auth) #74
• AppRole - Adding support for AppRole vault auth backend #105
• GitHub - Adding support for GitHub auth strategy for Vault datasources #80
• Token - Support for Vault datasources (app-id & token auth) #74

The user interface could look something like:

$ gomplate --vault-app-id=abcd1234 --vault-user-id=defg4567 \
    --datasource vault=https://my.vault.server:8200/v1/secret/supersecret \
    "my secret is: {{ (datasource "vault").data.value }}"
my secret is: supersecretvalue!