added external directives to Notifications, change case of keys in host, and update docs

docs/content/config/directives.md

@@ -0,0 +1,15 @@
+---
+title: "External Directives"
+weight: 2
+description: How to set up external directives.
+---
+
+External directives are for including data that should not be in the config file. The following directives are supported:
+
+- `%{file:path/to/file}%`
+- `%{env:ENV_VAR}%`
+- `%{vault:vault-key}%`
+
+See the docs of each command if the field is supported.
+
+If the file path does not begin with a `/`, the config file's directory will be used as the starting point.