Add config module

The config module supports adding configuration to the exporter via a config file. This supports adding authentication details in a config file so that /probe requests can specify authentication for endpoints

Signed-off-by: Joe Adams <github@joeadams.io>

config/testdata/config-bad-auth-module.yaml

@@ -0,0 +1,7 @@
+auth_modules:
+  foo:
+    pretendauth:
+      username: test
+      password: pass
+    options:
+      extra: "1"

config/testdata/config-bad-extra-field.yaml

@@ -0,0 +1,8 @@
+auth_modules:
+  foo:
+    userpass:
+      username: test
+      password: pass
+    options:
+      extra: "1"
+    doesNotExist: test

config/testdata/config-good.yaml

@@ -0,0 +1,8 @@
+auth_modules:
+  first:
+    type: userpass
+    userpass:
+      username: first
+      password: firstpass
+    options:
+      sslmode: disable